Same problem here. As far as I know, I followed the instructions to the t in integrating these two DBs – I have the blog running on one db, and the bb running on a second, but the users being “pulled” from the blog db. Once I did that, I lost all rights to access the backend. Have changed everything I can find on the backend to make this work, made edits in phpmyadmin to the user settings in the server, and even tried the plugin that was made available, Fix Admin Access – no avail.
Would love some other pointers if people have gotten buried on this. I’m not sure what else to pull together here.