I have exactly the same issue as described here:
https://forums.opera.com/topic/27865/opera-synchronization-failed

I reinstalled Windows and can't get back all my Opera stuff. https://sync.opera.com/web/ shows zero of every item.

Newly made account syncs immediately, showing green check mark, and doesn't have any problems.

I only use Opera on one device and it's this pc. I had Opera Sync enabled before reinstalling.

I had the same problem a half of year ago and it fixed itself somehow within few hours.
However as of now I'm waiting for 5 days already and all I see is orangish exclamation mark and "Signing in".
Few times, though, I got red cross icon telling me that something went wrong.

I would really like to get all my bookmarks back - had hundreds of them.