The passphrase can also be your (old) OS password if you are installing a new one.

We are talking about sync password or sync passphrase? Sync password is your Opera account password, the same that you use to login here in the forums (unless you have created a different account, while sync passphrase is the one that you create to encrypt your synced data. To reset the sync password, you go to your account login page and click on 'forgot password' (or something like this). To create a new passphrase, you need to reset your synced data and then go to the sync settings to disable passphrase or add a new one.

Do you have a backup?

I've signed into sync and its not downloading anything. No history, no bookmarks, nothing... Why?? How can I even see data and bookmarks? I've tried using Opera sync several times over the last year and it NEVER works... what the hell??

I have the same problem here, I have tried all old passwords and everything I have ever used as a password, yet none of them work. This has happened as I recently got a new laptop as old one was not working properly. I never set up a passphrase. This needs to be fixed.

Got the similar problem. (got just some dials, not all) However, it seems to me there's nobody interested in user's problems (after "recent" changes in Opera). It's a shame...