Confluence stopped creating thumbnails for uploaded PDF/PPTX files

[Gerald Schneider]

Recently one of our three Confluence instances stopped generating thumbnails for uploaded pdf files.

Excerpt from the log:

2024-05-07 09:51:57,062 INFO [http-nio-8090-exec-11] [synchrony.service.http.SynchronyRequestExecutor] execute Initiating request PUT http://127.0.0.1:8091/synchrony/v1/data/Example-3b6ece80-5d0e-31f3-ab38-f11704b0163a/confluence-50331658?state-at=@head&state-format=html HTTP/1.1 for 50331658
2024-05-07 09:52:09,637 WARN [http-nio-8090-exec-17] [internal.content.collab.ReconcileReferringContentTask] getShouldIncludeOwnContent Could not find shouldIncludeOwnContentMap for Content ID 84646070 but will reconcile anyway
-- url: /confluence/plugins/drag-and-drop/upload.action | userName: gerald | action: upload | referer: https://host.example.com/confluence/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: 22532f09db822c0c
2024-05-07 09:52:09,644 WARN [document-conversion-sandbox-launcher-2] [impl.util.sandbox.SandboxProcess] startIfDead Sandbox 0 has died
-- url: /confluence/rest/documentConversion/latest/conversion/thumbnail/results | userName: gerald | referer: https://host.example.com/confluence/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: eda882a0f99a205d
2024-05-07 09:52:09,644 INFO [http-nio-8090-exec-17] [synchrony.service.http.SynchronyRequestExecutor] execute Initiating request PUT http://127.0.0.1:8091/synchrony/v1/data/Example-3b6ece80-5d0e-31f3-ab38-f11704b0163a/confluence-50331658?state-at=@head&state-format=html HTTP/1.1 for 50331658
2024-05-07 09:52:09,645 WARN [document-conversion-sandbox-launcher-2] [impl.util.sandbox.SandboxProcess] startIfDead Attempting to restart the sandbox 0
-- url: /confluence/rest/documentConversion/latest/conversion/thumbnail/results | userName: gerald | referer: https://host.example.com/confluence/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: eda882a0f99a205d
2024-05-07 09:52:12,410 INFO [http-nio-8090-exec-2] [synchrony.service.http.SynchronyRequestExecutor] execute Initiating request PUT http://127.0.0.1:8091/synchrony/v1/data/Example-3b6ece80-5d0e-31f3-ab38-f11704b0163a/confluence-50331658?state-at=@head&state-format=html HTTP/1.1 for 50331658
2024-05-07 09:53:11,444 WARN [Caesium-1-4] [impl.schedule.caesium.JobRunnerWrapper] runJob Scheduled job com.balsamiq.scheduler.JobRunner_BAS_Gardening#com.balsamiq.confluence.plugins.mockups.Scheduler_2 failed with response JobRunnerResponse[runOutcome=FAILED,message='doYourStuffHere failed: service proxy has been destroyed']
2024-05-07 09:54:11,994 WARN [sandbox-terminator] [impl.util.sandbox.SandboxLocalProcessPool] lambda$startTerminator$0 Request has taken 122349ms exceeds l
imit 120000ms terminating sandbox
-- url: /confluence/plugins/drag-and-drop/upload.action | userName: gerald | action: upload | referer: https://host.example.com/confluence/pag
es/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: 0620767445f8d8c4
2024-05-07 09:54:12,012 WARN [document-conversion-sandbox-launcher-3] [impl.util.sandbox.SandboxProcess] startIfDead Sandbox 0 has died
-- url: /confluence/rest/documentConversion/latest/conversion/thumbnail/results | userName: gerald | referer: https://host.example.com/conflue
nce/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: 192e9486318c337d
2024-05-07 09:54:12,013 WARN [document-conversion-sandbox-launcher-3] [impl.util.sandbox.SandboxProcess] startIfDead Attempting to restart the sandbox 0
-- url: /confluence/rest/documentConversion/latest/conversion/thumbnail/results | userName: gerald | referer: https://host.example.com/conflue
nce/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: 192e9486318c337d
2024-05-07 09:54:12,013 WARN [sandbox-terminator] [impl.util.sandbox.SandboxLocalProcessPool] lambda$startTerminator$0 Request has taken 121104ms exceeds l
imit 120000ms terminating sandbox
-- url: /confluence/plugins/drag-and-drop/upload.action | userName: gerald | action: upload | referer: https://host.example.com/confluence/pag
es/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: 0620767445f8d8c4
2024-05-07 09:54:12,015 WARN [ForkJoinPool.commonPool-worker-3] [plugins.conversion.impl.LocalFileSystemConversionResultSupplier] lambda$performConversionI
nSandbox$2 Error when performing conversion com.atlassian.confluence.plugins.conversion.impl.LocalFileSystemConversionResultSupplier$3@660eb825[attachment=
/download/attachments/50331658/2024-04-18%20Report%20SC.pdf?version=1&modificationDate=1715068329507&api=v2,conversionType=THUMBNAIL] in the sandbox
java.util.concurrent.CompletionException: com.atlassian.confluence.util.sandbox.SandboxTimeoutException: Sandbox request has been killed because it exceede
d time limit of 120 seconds
at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:315)
at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:320)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1770)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: com.atlassian.confluence.util.sandbox.SandboxTimeoutException: Sandbox request has been killed because it exceeded time limit of 120 seconds
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.tryToThrowSpecificException(SandboxProcess.java:171)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.execute(SandboxProcess.java:139)
at com.atlassian.confluence.impl.util.sandbox.SandboxLocalProcessPool.execute(SandboxLocalProcessPool.java:107)
at com.atlassian.confluence.impl.util.sandbox.ConfluenceSandboxPoolFactory$LazySandbox.execute(ConfluenceSandboxPoolFactory.java:70)
at com.atlassian.confluence.impl.util.sandbox.DefaultSandbox.execute(DefaultSandbox.java:41)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.LocalDocumentConversionSandbox.execute(LocalDocumentConversionSandbox.java:66)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.LocalDocumentConversionSandbox.execute(LocalDocumentConversionSandbox.java:59)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.DelegatingDocumentConversionSandbox.execute(DelegatingDocumentConversionSandbox.java:47)
at com.atlassian.confluence.plugins.conversion.impl.LocalFileSystemConversionResultSupplier.lambda$performConversionInSandbox$1(LocalFileSystemConversionResultSupplier.java:227)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768)
... 3 more
Caused by: java.io.EOFException: error deserialize sandbox message
at com.atlassian.confluence.impl.util.sandbox.SandboxMessage.receiveMessage(SandboxMessage.java:50)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.receiveMessage(SandboxProcess.java:227)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.execute(SandboxProcess.java:136)
... 11 more
2024-05-07 09:54:12,023 WARN [ForkJoinPool.commonPool-worker-3] [plugins.conversion.impl.FileSystemConversionState] markAsError Creating error file: /var/atlassian/application-data/confluence/shared-home/dcl-thumbnail/158/81/50331658/352944134_1.err
2024-05-07 09:54:12,036 WARN [document-conversion-sandbox-launcher-2] [impl.util.sandbox.SandboxProcess] startIfDead Sandbox 1 has died
-- url: /confluence/rest/documentConversion/latest/conversion/thumbnail/results | userName: gerald | referer: https://host.example.com/confluence/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: eda882a0f99a205d
2024-05-07 09:54:12,036 WARN [document-conversion-sandbox-launcher-2] [impl.util.sandbox.SandboxProcess] startIfDead Attempting to restart the sandbox 1
-- url: /confluence/rest/documentConversion/latest/conversion/thumbnail/results | userName: gerald | referer: https://host.example.com/confluence/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: eda882a0f99a205d
2024-05-07 09:54:12,038 WARN [ForkJoinPool.commonPool-worker-3] [plugins.conversion.impl.LocalFileSystemConversionResultSupplier] lambda$performConversionInSandbox$2 Error when performing conversion com.atlassian.confluence.plugins.conversion.impl.LocalFileSystemConversionResultSupplier$3@16de519a[attachment=/download/attachments/50331658/2024-04-18%20Report%20SC.pdf?version=1&modificationDate=1715068329507&api=v2,conversionType=THUMBNAIL] in the sandbox
java.util.concurrent.CompletionException: com.atlassian.confluence.util.sandbox.SandboxTimeoutException: Sandbox request has been killed because it exceeded time limit of 120 seconds
at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:315)
at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:320)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1770)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: com.atlassian.confluence.util.sandbox.SandboxTimeoutException: Sandbox request has been killed because it exceeded time limit of 120 seconds
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.tryToThrowSpecificException(SandboxProcess.java:171)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.execute(SandboxProcess.java:139)
at com.atlassian.confluence.impl.util.sandbox.SandboxLocalProcessPool.execute(SandboxLocalProcessPool.java:107)
at com.atlassian.confluence.impl.util.sandbox.ConfluenceSandboxPoolFactory$LazySandbox.execute(ConfluenceSandboxPoolFactory.java:70)
at com.atlassian.confluence.impl.util.sandbox.DefaultSandbox.execute(DefaultSandbox.java:41)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.LocalDocumentConversionSandbox.execute(LocalDocumentConversionSandbox.java:66)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.LocalDocumentConversionSandbox.execute(LocalDocumentConversionSandbox.java:59)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.DelegatingDocumentConversionSandbox.execute(DelegatingDocumentConversionSandbox.java:47)
at com.atlassian.confluence.plugins.conversion.impl.LocalFileSystemConversionResultSupplier.lambda$performConversionInSandbox$1(LocalFileSystemConversionResultSupplier.java:227)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768)
... 3 more
Caused by: java.io.EOFException: error deserialize sandbox message
at com.atlassian.confluence.impl.util.sandbox.SandboxMessage.receiveMessage(SandboxMessage.java:50)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.receiveMessage(SandboxProcess.java:227)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.execute(SandboxProcess.java:136)
... 11 more
2024-05-07 09:54:12,038 WARN [ForkJoinPool.commonPool-worker-3] [plugins.conversion.impl.FileSystemConversionState] markAsError Creating error file: /var/atlassian/application-data/confluence/shared-home/dcl-thumbnail/158/81/50331658/352944134_1.err
2024-05-07 09:54:15,037 WARN [sandbox-terminator] [impl.util.sandbox.SandboxLocalProcessPool] lambda$startTerminator$0 Request has taken 123143ms exceeds limit 120000ms terminating sandbox
-- url: /confluence/plugins/drag-and-drop/upload.action | userName: gerald | action: upload | referer: https://host.example.com/confluence/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: 0620767445f8d8c4
2024-05-07 09:54:15,055 WARN [document-conversion-sandbox-launcher-4] [impl.util.sandbox.SandboxProcess] startIfDead Sandbox 0 has died
-- url: /confluence/rest/api/content/50331658 | userName: gerald | referer: https://host.example.com/confluence/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: 54efb0e50b050e75
2024-05-07 09:54:15,055 WARN [document-conversion-sandbox-launcher-4] [impl.util.sandbox.SandboxProcess] startIfDead Attempting to restart the sandbox 0
-- url: /confluence/rest/api/content/50331658 | userName: gerald | referer: https://host.example.com/confluence/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: 54efb0e50b050e75
2024-05-07 09:54:15,061 WARN [ForkJoinPool.commonPool-worker-3] [plugins.conversion.impl.LocalFileSystemConversionResultSupplier] lambda$performConversionInSandbox$2 Error when performing conversion com.atlassian.confluence.plugins.conversion.impl.LocalFileSystemConversionResultSupplier$3@36491c8f[attachment=/download/attachments/50331658/2024-04-18%20Report%20SC.pdf?version=1&modificationDate=1715068329507&api=v2,conversionType=THUMBNAIL] in the sandbox
java.util.concurrent.CompletionException: com.atlassian.confluence.util.sandbox.SandboxTimeoutException: Sandbox request has been killed because it exceeded time limit of 120 seconds
at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:315)
at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:320)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1770)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: com.atlassian.confluence.util.sandbox.SandboxTimeoutException: Sandbox request has been killed because it exceeded time limit of 120 seconds
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.tryToThrowSpecificException(SandboxProcess.java:171)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.execute(SandboxProcess.java:139)
at com.atlassian.confluence.impl.util.sandbox.SandboxLocalProcessPool.execute(SandboxLocalProcessPool.java:107)
at com.atlassian.confluence.impl.util.sandbox.ConfluenceSandboxPoolFactory$LazySandbox.execute(ConfluenceSandboxPoolFactory.java:70)
at com.atlassian.confluence.impl.util.sandbox.DefaultSandbox.execute(DefaultSandbox.java:41)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.LocalDocumentConversionSandbox.execute(LocalDocumentConversionSandbox.java:66)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.LocalDocumentConversionSandbox.execute(LocalDocumentConversionSandbox.java:59)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.DelegatingDocumentConversionSandbox.execute(DelegatingDocumentConversionSandbox.java:47)
at com.atlassian.confluence.plugins.conversion.impl.LocalFileSystemConversionResultSupplier.lambda$performConversionInSandbox$1(LocalFileSystemConversionResultSupplier.java:227)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768)
... 3 more
Caused by: java.io.IOException: Stream closed
at java.base/java.io.BufferedInputStream.getBufIfOpen(BufferedInputStream.java:168)
at java.base/java.io.BufferedInputStream.fill(BufferedInputStream.java:212)
at java.base/java.io.BufferedInputStream.read(BufferedInputStream.java:263)
at java.base/java.io.DataInputStream.readInt(DataInputStream.java:382)
at com.atlassian.confluence.impl.util.sandbox.SandboxMessage.receiveMessage(SandboxMessage.java:43)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.receiveMessage(SandboxProcess.java:227)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.execute(SandboxProcess.java:136)
... 11 more
2024-05-07 09:54:15,061 WARN [ForkJoinPool.commonPool-worker-3] [plugins.conversion.impl.FileSystemConversionState] markAsError Creating error file: /var/atlassian/application-data/confluence/shared-home/dcl-thumbnail/158/81/50331658/352944134_1.err
2024-05-07 09:54:28,038 WARN [sandbox-terminator] [impl.util.sandbox.SandboxLocalProcessPool] lambda$startTerminator$0 Request has taken 137445ms exceeds limit 120000ms terminating sandbox
-- url: /confluence/plugins/drag-and-drop/upload.action | userName: gerald | action: upload | referer: https://host.example.com/confluence/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: 0620767445f8d8c4
2024-05-07 09:54:28,060 WARN [document-conversion-sandbox-launcher-3] [impl.util.sandbox.SandboxProcess] startIfDead Sandbox 0 has died
-- url: /confluence/rest/documentConversion/latest/conversion/thumbnail/results | userName: gerald | referer: https://host.example.com/confluence/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: 192e9486318c337d
2024-05-07 09:54:28,060 WARN [document-conversion-sandbox-launcher-3] [impl.util.sandbox.SandboxProcess] startIfDead Attempting to restart the sandbox 0
-- url: /confluence/rest/documentConversion/latest/conversion/thumbnail/results | userName: gerald | referer: https://host.example.com/confluence/pages/resumedraft.action?draftId=84646070&draftShareId=dfeae20b-393a-423f-aa08-400c195dcc52& | traceId: 192e9486318c337d
2024-05-07 09:54:28,066 WARN [ForkJoinPool.commonPool-worker-3] [plugins.conversion.impl.LocalFileSystemConversionResultSupplier] lambda$performConversionInSandbox$2 Error when performing conversion com.atlassian.confluence.plugins.conversion.impl.LocalFileSystemConversionResultSupplier$3@11fc66d6[attachment=/download/attachments/50331658/2024-04-18%20Report%20SC.pdf?version=1&modificationDate=1715068329507&api=v2,conversionType=THUMBNAIL] in the sandbox
java.util.concurrent.CompletionException: com.atlassian.confluence.util.sandbox.SandboxTimeoutException: Sandbox request has been killed because it exceeded time limit of 120 seconds
at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:315)
at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:320)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1770)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: com.atlassian.confluence.util.sandbox.SandboxTimeoutException: Sandbox request has been killed because it exceeded time limit of 120 seconds
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.tryToThrowSpecificException(SandboxProcess.java:171)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.execute(SandboxProcess.java:139)
at com.atlassian.confluence.impl.util.sandbox.SandboxLocalProcessPool.execute(SandboxLocalProcessPool.java:107)
at com.atlassian.confluence.impl.util.sandbox.ConfluenceSandboxPoolFactory$LazySandbox.execute(ConfluenceSandboxPoolFactory.java:70)
at com.atlassian.confluence.impl.util.sandbox.DefaultSandbox.execute(DefaultSandbox.java:41)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.LocalDocumentConversionSandbox.execute(LocalDocumentConversionSandbox.java:66)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.LocalDocumentConversionSandbox.execute(LocalDocumentConversionSandbox.java:59)
at com.atlassian.confluence.plugins.conversion.impl.sandbox.DelegatingDocumentConversionSandbox.execute(DelegatingDocumentConversionSandbox.java:47)
at com.atlassian.confluence.plugins.conversion.impl.LocalFileSystemConversionResultSupplier.lambda$performConversionInSandbox$1(LocalFileSystemConversionResultSupplier.java:227)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768)
... 3 more
Caused by: java.io.EOFException: error deserialize sandbox message
at com.atlassian.confluence.impl.util.sandbox.SandboxMessage.receiveMessage(SandboxMessage.java:50)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.receiveMessage(SandboxProcess.java:227)
at com.atlassian.confluence.impl.util.sandbox.SandboxProcess.execute(SandboxProcess.java:136)
... 11 more
2024-05-07 09:54:28,067 WARN [ForkJoinPool.commonPool-worker-3] [plugins.conversion.impl.FileSystemConversionState] markAsError Creating error file: /var/atlassian/application-data/confluence/shared-home/dcl-thumbnail/158/81/50331658/352944134_1.err

I already went through these KB:

- https://confluence.atlassian.com/confkb/pdf-macro-failed-to-generate-the-thumbnail-image-1079348159.html

converting documents is not an option and IMO disabling the memory guard is not really an option either.

Instead I went with this KB article:

- https://confluence.atlassian.com/doc/document-conversion-for-confluence-data-center-952624318.html

and increased the timeout from the default 30 first to 60 and later to 120 seconds. This didn't help either.

JAVA_TOOL_OPTIONS: ' -Duser.timezone="Europe/Berlin" -Dconfluence.document.conversion.fontpath=/usr/share/fonts/truetype/msttcorefonts/ -Dconversion.sandbox.java.options=-Dconfluence.document.conversion.slides.convert.timeout=120 -Ddocument.conversion.sandbox.request.time.limit.secs=120 '

The .err files mentioned in the error messages are all empty, no help from there either.

The documents I used to test are a converted PPTX file with 6 slides and a pptx file directly, both files only around 240KB in size, so I don't think the file size is an issue. All three instances are running in the official Docker container with exactly the same options.

I also already tried uploading files in safe mode, which had the same result. So no installed add-on is responsible either.

Sadly, the problem only appears on the instance that is running a community license, so I can't open a support request for it.

Are there more log files I can take a look at, or can I increase the log verbosity for the process that is responsible for the thumbnail generation?