I just figured it out. I had to go into the opera account page, and "reset passphrase", whatever that is. Then I were able to log in on my phone, and enable sync
Although the sync server were cleaned out by reseting the passphrase, I don't think I'll loose any data, as everything gets restored when I sync up all my devices again. (I hope)
Hope I didn't look too dumb in the end. I've been using opera as my main browser on all my devices for some 7 years now, but I've never had many issues like this before, which I'm quite glad for, actually.