Elasticsearch fails to recreate index after upgrade

[Sys Admin]

I've upgraded Bitbucket Server from 5.15 to 6.6.1, and Git access and web interface all appears fine.

Elasticsearch though does have an issue, where it is unable to rebuild the index post upgrade, and so all searches return no result.

The symptoms are ridiculously similar to this question:
https://community.atlassian.com/t5/Bitbucket-questions/Elasticsearche-error-after-upgrade/qaq-p/1113655

I have attempted to trigger a reindex, with the same error reported (included below).

Have also tried deleting the index (%BITBUCKET_HOME%/shared/search/data) and restarting Bitbucket, and it again attempts to reindex but then fails in the same way.

2019-09-24 10:50:43,098 INFO  [Caesium-1-2]  c.a.b.i.s.i.jobs.StartupChecksJob Running startup jobs for search
2019-09-24 10:50:43,125 INFO  [Caesium-1-2]  c.a.b.i.s.i.u.DefaultUpgradeService Executing upgrade task:[Create indexes for each mapping, this is required for ES 6.x support.]
2019-09-24 10:50:49,165 ERROR [Caesium-1-2]  c.a.b.i.s.i.u.UpgradeTaskV3Point0 Failed to recreate index
java.util.concurrent.ExecutionException: java.net.SocketTimeoutException
at java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:357)
at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1895)
at rx.internal.operators.OnSubscribeToObservableFuture$ToObservableFuture.call(OnSubscribeToObservableFuture.java:74)
at rx.internal.operators.OnSubscribeToObservableFuture$ToObservableFuture.call(OnSubscribeToObservableFuture.java:43)
at rx.Observable.unsafeSubscribe(Observable.java:10327)
at rx.internal.operators.OnSubscribeMap.call(OnSubscribeMap.java:48)
at rx.internal.operators.OnSubscribeMap.call(OnSubscribeMap.java:33)
at rx.Observable.unsafeSubscribe(Observable.java:10327)
at rx.internal.operators.OperatorMerge$MergeSubscriber.onNext(OperatorMerge.java:248)
at rx.internal.operators.OperatorMerge$MergeSubscriber.onNext(OperatorMerge.java:148)
at rx.internal.operators.OnSubscribeFromArray$FromArrayProducer.fastPath(OnSubscribeFromArray.java:76)
at rx.internal.operators.OnSubscribeFromArray$FromArrayProducer.request(OnSubscribeFromArray.java:58)
at rx.Subscriber.setProducer(Subscriber.java:211)
at rx.internal.operators.OnSubscribeFromArray.call(OnSubscribeFromArray.java:32)
at rx.internal.operators.OnSubscribeFromArray.call(OnSubscribeFromArray.java:24)
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:48)
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:30)
at rx.Observable.unsafeSubscribe(Observable.java:10327)
at rx.internal.operators.OperatorMerge$MergeSubscriber.onNext(OperatorMerge.java:248)
at rx.internal.operators.OperatorMerge$MergeSubscriber.onNext(OperatorMerge.java:148)
at rx.internal.operators.OnSubscribeMap$MapSubscriber.onNext(OnSubscribeMap.java:77)
at rx.internal.producers.SingleProducer.request(SingleProducer.java:65)
at rx.Subscriber.setProducer(Subscriber.java:211)
at rx.internal.operators.OnSubscribeMap$MapSubscriber.setProducer(OnSubscribeMap.java:102)
at rx.internal.operators.OnSubscribeToObservableFuture$ToObservableFuture.call(OnSubscribeToObservableFuture.java:75)
at rx.internal.operators.OnSubscribeToObservableFuture$ToObservableFuture.call(OnSubscribeToObservableFuture.java:43)
at rx.Observable.unsafeSubscribe(Observable.java:10327)
at rx.internal.operators.OnSubscribeMap.call(OnSubscribeMap.java:48)
at rx.internal.operators.OnSubscribeMap.call(OnSubscribeMap.java:33)
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:48)
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:30)
at rx.Observable.subscribe(Observable.java:10423)
at rx.Observable.subscribe(Observable.java:10390)
at rx.Observable.subscribe(Observable.java:10271)
at com.atlassian.bitbucket.internal.search.indexing.util.Observables.consume(Observables.java:34)
at com.atlassian.bitbucket.internal.search.indexing.upgrade.UpgradeTaskV3Point0.execute(UpgradeTaskV3Point0.java:39)
at com.atlassian.bitbucket.internal.search.indexing.upgrade.DefaultUpgradeService.upgrade(DefaultUpgradeService.java:45)
at com.atlassian.bitbucket.internal.search.indexing.IndexingSynchronizationService.synchronizeMapping(IndexingSynchronizationService.java:112)
at com.atlassian.bitbucket.internal.search.indexing.IndexingSynchronizationService.synchronizeStores(IndexingSynchronizationService.java:84)
at com.atlassian.bitbucket.internal.search.indexing.jobs.StartupChecksJob.run(StartupChecksJob.java:80)
at com.atlassian.bitbucket.internal.search.common.cluster.ClusterJobRunner.runJob(ClusterJobRunner.java:81)
at com.atlassian.scheduler.core.JobLauncher.runJob(JobLauncher.java:134)
at com.atlassian.scheduler.core.JobLauncher.launchAndBuildResponse(JobLauncher.java:106)
at com.atlassian.scheduler.core.JobLauncher.launch(JobLauncher.java:90)
at com.atlassian.scheduler.caesium.impl.CaesiumSchedulerService.launchJob(CaesiumSchedulerService.java:435)
at com.atlassian.scheduler.caesium.impl.CaesiumSchedulerService.executeClusteredJob(CaesiumSchedulerService.java:430)
at com.atlassian.scheduler.caesium.impl.CaesiumSchedulerService.executeClusteredJobWithRecoveryGuard(CaesiumSchedulerService.java:454)
at com.atlassian.scheduler.caesium.impl.CaesiumSchedulerService.executeQueuedJob(CaesiumSchedulerService.java:382)
at com.atlassian.scheduler.caesium.impl.SchedulerQueueWorker.executeJob(SchedulerQueueWorker.java:66)
at com.atlassian.scheduler.caesium.impl.SchedulerQueueWorker.executeNextJob(SchedulerQueueWorker.java:60)
at com.atlassian.scheduler.caesium.impl.SchedulerQueueWorker.run(SchedulerQueueWorker.java:35)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.net.SocketTimeoutException: null
at org.apache.http.nio.protocol.HttpAsyncRequestExecutor.timeout(HttpAsyncRequestExecutor.java:375)
at org.apache.http.impl.nio.client.InternalIODispatch.onTimeout(InternalIODispatch.java:92)
at org.apache.http.impl.nio.client.InternalIODispatch.onTimeout(InternalIODispatch.java:39)
at org.apache.http.impl.nio.reactor.AbstractIODispatch.timeout(AbstractIODispatch.java:175)
at org.apache.http.impl.nio.reactor.BaseIOReactor.sessionTimedOut(BaseIOReactor.java:263)
at org.apache.http.impl.nio.reactor.AbstractIOReactor.timeoutCheck(AbstractIOReactor.java:492)
at org.apache.http.impl.nio.reactor.BaseIOReactor.validate(BaseIOReactor.java:213)
at org.apache.http.impl.nio.reactor.AbstractIOReactor.execute(AbstractIOReactor.java:280)
at org.apache.http.impl.nio.reactor.BaseIOReactor.execute(BaseIOReactor.java:104)
at org.apache.http.impl.nio.reactor.AbstractMultiworkerIOReactor$Worker.run(AbstractMultiworkerIOReactor.java:588)
... 1 common frames omitted
2019-09-24 10:50:49,167 ERROR [Caesium-1-2]  c.a.b.i.s.i.u.UpgradeTaskV3Point0 Failed to recreate indexes: ''[java.net.SocketTimeoutException]''
2019-09-24 10:50:49,167 ERROR [Caesium-1-2]  c.a.b.i.s.i.u.DefaultUpgradeService Upgrade task failed:[Create indexes for each mapping, this is required for ES 6.x support.]
com.atlassian.bitbucket.internal.search.indexing.exceptions.IndexException: Failed to recreate index: [java.net.SocketTimeoutException]
at com.atlassian.bitbucket.internal.search.indexing.upgrade.UpgradeTaskV3Point0.execute(UpgradeTaskV3Point0.java:52)
at com.atlassian.bitbucket.internal.search.indexing.upgrade.DefaultUpgradeService.upgrade(DefaultUpgradeService.java:45)
at com.atlassian.bitbucket.internal.search.indexing.IndexingSynchronizationService.synchronizeMapping(IndexingSynchronizationService.java:112)
at com.atlassian.bitbucket.internal.search.indexing.IndexingSynchronizationService.synchronizeStores(IndexingSynchronizationService.java:84)
at com.atlassian.bitbucket.internal.search.indexing.jobs.StartupChecksJob.run(StartupChecksJob.java:80)
at com.atlassian.bitbucket.internal.search.common.cluster.ClusterJobRunner.runJob(ClusterJobRunner.java:81)
at com.atlassian.scheduler.core.JobLauncher.runJob(JobLauncher.java:134)
at com.atlassian.scheduler.core.JobLauncher.launchAndBuildResponse(JobLauncher.java:106)
at com.atlassian.scheduler.core.JobLauncher.launch(JobLauncher.java:90)
at com.atlassian.scheduler.caesium.impl.CaesiumSchedulerService.launchJob(CaesiumSchedulerService.java:435)
at com.atlassian.scheduler.caesium.impl.CaesiumSchedulerService.executeClusteredJob(CaesiumSchedulerService.java:430)
at com.atlassian.scheduler.caesium.impl.CaesiumSchedulerService.executeClusteredJobWithRecoveryGuard(CaesiumSchedulerService.java:454)
at com.atlassian.scheduler.caesium.impl.CaesiumSchedulerService.executeQueuedJob(CaesiumSchedulerService.java:382)
at com.atlassian.scheduler.caesium.impl.SchedulerQueueWorker.executeJob(SchedulerQueueWorker.java:66)
at com.atlassian.scheduler.caesium.impl.SchedulerQueueWorker.executeNextJob(SchedulerQueueWorker.java:60)
at com.atlassian.scheduler.caesium.impl.SchedulerQueueWorker.run(SchedulerQueueWorker.java:35)
at java.lang.Thread.run(Thread.java:748)
2019-09-24 10:50:49,168 ERROR [Caesium-1-2]  c.a.b.i.s.i.IndexingSynchronizationService Upgrading index failed, unable to continue, operator intervention is required.
2019-09-24 10:50:49,169 WARN  [Caesium-1-2]  c.a.b.i.s.c.cluster.ClusterJobRunner Job execution failed. Rescheduling. Job key: com.atlassian.bitbucket.internal.search.indexing.jobs.StartupChecksJob  Job id: 4dccfb57-b78e-4d26-9c9c-eb3a37825566