Should have said, I actually did:
Maintenance mode on
Backed up DB
Deleted corrupt calendar entry
Maintenance mode off
(In case anyone needs to do the same - always take a backup and always do it while in maintenance mode)
I use Davdroid to sync with Owncloud. I do not sync tasks and did not install OpenTasks. Then, I get the following sync error:
08-01 13:54:11.269 10623 11056 E ActivityThread: Failed to find provider info for org.dmfs.tasks 08-01 13:54:11.279 659 697 D SyncManager: failed sync operation Owncloud u0 (bitfire.at.davdroid), org.dmfs.tasks, USER, currentRunTime 5133228, EXPEDITED, reason: 10119, SyncResult: databaseError: true stats  08-01 13:54:11.282 659 697 D SyncManager: not retrying sync operation because the error is a hard error: Owncloud u0 (bitfire.at.davdroid), org.dmfs.tasks, USER, currentRunTime 5183103, reason: 10119
Only when I manually change the synchronization interval in Davdroid for tasks from every 240 minutes to manually, the synchronization works without errors.
Yes, I am using Android 6.0.