@rcb There's no way to make things work differently.
When you use a passphrase, all your synced data is encrypted using that passphrase. If you don't remember it, then your data can't be decrypted and, therefore, can't be used.
So, the only way to have a new passphrase is by removing all encrypted synced data.
@mmichel It's probably part of one those options. I would guess settings.
You guessed well.
So my guess is now that I will sync settings to initialize them, but will stop that just after to gain workspace isolation.
Far from perfect...
@jfk-helpdesk If www,sync.opera.com says that there are no synced passwords and didn't make a backup by exporting the passwords in Opera for desktops, then probably there's nothing that can be done.