It shouldn't be dependent on if you are logged in or not.
I am still testing the issue but what I can say so far - you are right it it's not depending on if I am logged in or not.
It seems to depend on the password manager window. When the before mentioned login page is opened, Opera ads the password manager window on top of the page. As long as this password manager window is open in the background the bookmark editing window is corrupted at any tabbed website. As soon as the password manager window is closed (by logging in or just cancelling the login in the password manager) all bookmark editing is back to normal.