Our site has switched to the new Confluence editor, which may or may not be related to this problem.
I just tried creating a new diagram. It worked fine when previewing in the dialog but when I clicked Insert, creating the attachment failed. Looking at the Network tab in Chrome Dev Tools, I see that it POSTed to: /wiki/rpc/json-rpc/confluenceservice-v2/addAttachment and the response was 501 ("Not Implemented").
I have created PlantUML attachments in the past, but that was before our site switched to the new editor.
I haven't found any similar questions yet... help?