Hello,
I have 2 macOS laptop v2.5.12 app syncing perfectly fine with Docker/Joplin Server 2.5.5. But I am having problem with Android app v2.6.3 (Error 412 Precondition Failed).
SOLUTION: I have updated Joplin Server v2.6.10, and not Android app v2.6.3 now works well again.
Thanks!