Disabling Sandbox for an app doesn't work

Trying to use Facebook Connect allows users to log into our new vBulletin board 4.1.11. It works great, but only for people added admins, testers, etc. to the app. As soon as a user is removed from Admins / Testers, they receive the message "An error occurred. Please try later" when trying to login.

It looks like the application is "stuck" in Sandbox mode, although I disabled Sandbox mode.

Thank!