I got the same problem here probabbly, but I did this:
- Open the Sync Settings
- Disable all sync features (bookmark, password, settings, history)
- Restarted the Opera
- Open the Sync Settings
- Enabled the Bookmark
- Restarted the Opera
- Wait some seconds, and my bookmarks as sucessfully synced
- Open the Sync Settings
- Disabled the Bookmark Sync
- Enabled the Password Sync
- Repeat the bookmark process
Then I visited the https://sync.opera.com/web/ and reseted my account data,
After this I restarted the browser again, to he detect the sync logout.
After everyhing I did login into my account and enabled all sync features, and now he is working.
I hope this work to you too.