We recently deactivated the bbPress plugin because we don’t use it anymore. Immediately following that, no admin users can get into the dashboard without hitting the infamous:
"You do not have sufficient permissions to access this page?"
We tried removing ALL plugins to rule out a dependance from one of those, but it didn’t work.
For the specific case of bbPress, it adds new user roles and capabilities, like the “Keymaster” role. Those users who had a role that was bbPress specific won’t have access to anything anymore, because after you removed the plugin, the role no longer exists.
Some users have reported that using the “User Role Editor” plugin and resetting their roles to default have fixed this issue:
http://bbpress.org/forums/topic/you-do-not-have-sufficient-permissions-to-access-this-page-3/
If you have any admin that can login, he can manually reset the roles of affected users by editing their user profile and giving them the proper role on the site.