1. Open the wallet window.
2. Click on the cog icon (top right).
3. Click 'Import wallet'.
4. Re-enter your 12 word secret seed.
5. Enter a new password to secure it on the next screen.

The above worked for me on Desktop.

I too was confronted with a password that I am suspicious I never set. I could use on mobile with fingerprint, but not on desktop without the unknown password.

This password is NOT the same password as your Opera account.