A migration bug was found:
https://github.com/owncloud/client/issues/9019
If you’re already on 2.9.0, you’ll see a file like .sync_286ccd77d0ee.db and another .sync_journal.db in each sync directory. You need to manually remove the ugly one .sync_XXXXXXXX.db, then the error goes away.
Auto-updater got stopped, fix will be in a quick 2.9.1. But if it already happened, you need to fix manually.