I have the same problem. I know my username and my password, but not the verification code. Not even the 8-digit backup code. All I can do is create a new account and use that one instead. I can't even log in with it on my mobile app.
@leocg Thank you, but who on earth came up with the idea of hiding it there? On the Synchronization page, it says "Do not remember your key synchronization password (passphrase)? Reset Passphrase". Not really the place you would look when you are told to click on the "Reset sync data button"...
At least it seems to have worked, I deleted it and have restarted a few times now after resyncing my devices so thank you!
However, this really leads to continued doubts about choosing Opera on a permanent basis. I couldn't even start Opera on any of my computers and the only way to solve the problem was to clear the synced data from a different browser. That means losing all bookmarks on all synced computers and the only reason I still have mine is that I could re-import them from Firefox, which is where I got them in the first place.