I have an application system and a panel running on another server from which I access the wp mysql database and change the users groupid. This change reflects on their profile but they cannot access the forums that the group should have access to. If I go to their profile and click "save changes" manually it does work, synchronizing users from the wordpress panel doesn't fix this either. Any idea on how this could be fixed? Thanks!
Found the solution: disable member cache as described by Robert here: