I have the same problem....
Joomla 1.5.8 and latest version of CB.
Search for over an hour here on the forum, but nothing solved my problem.
I do have Fire Board Forum installed.. disabled it, but that doesn't seem to help.
Tried SEF on and Off...
Tried to make new account through CB_login, but after activation, still get the same pop-up.
....?
....?