@_ck_ Sorry missed about the WPMU, I´m on WP 2.6 and having theese problems.
I think I might have the same problem. I can log in an post with one admin user. This user can post both on WP and BBP. If I add another user. It can semi login to WP but not to BBP. If I change this user to an admin nothing happens, the user cannot do anything more than before.
