@lneitan I had a similar problem.
А virus ate my operating system and I lost my sync password. Now, in an attempt to restore my bookmarks, I followed the path recommended by Opera and deleted everything.
In general, this is a serious criticism of Opera, because I already found a warning about it on the page https://help.opera.com/en/account/#password with FAQ after this, and there is no feedback on the question. In addition, it was all destroyed in one click. And no confirmation of actions, nor the ability to cancel the decision.
It's useless to talk about it, because nothing can be changed, but it's still very sad.