As test, log in to a Sync in a test standalone installation. Does it happen there too? If so, delete that standalone installation and make another one. Then log in to Sync in that new one, but choose to sync everything but settings. Any difference?
If things work fine in the test standalone installation, you might just have to start with a new profile for your regular Opera GX. Or, first try to delete the "Sync Data" folder in the profile folder while Opera is closed and then log back into Sync.
(When messing with Sync, always make sure you have backups of your stuff.)