I registered. The forum registration was forced through WordPress. I logged in at WordPress. I appear logged in at the forum. When I try page two, I got the login box from WordPress as you experienced.
When I tried I tried loggin in at bb-login.php with the password that was mailed out by WordPress, it said “incorrect password.”
So, I’d say it has something to do with the integration.
Has it ever worked before and this is new, or since installation it’s always been like this?