I'm also having this issue today. However, my sync broke completely during some update, it's not syncing bookmarks nor tabs properly anymore, there are leftover tabs from few months ago. I tried resetting sync as well (on web and locally), didn't help.
I'm using Opera on multiple machines, Windows, macOS and Linux.SCR-20230328-nbzu.png
@embcla I had the same issue - did clean reinstall and it went through OK; during the process I chose the "existing user" and after puting my credencials and password I was asked for a passphrase after choosing what items I want to sync. For me - case closed.
@vonnis If you reset your passphrase in www.sync.opera.com, then your synced data was removed as you were warned and there is no way to recover it as said in the several other topics about the exactly same issue that you could have just read.