When a user that is not an administrator tries to log in, the response is status 500 server error.
Also tried User Switching with the same fatal result trying to switch.
I have turned off a lot of security features provided by this plugin just to see if I can track down what causes this, with no luck.
I would believe that this plugin would never triggers a status 500 deliberately, but uses some other status to deny the request.
Deactivating the plugin immediately restores proper login for my editors and contributors.
Server runs PHP 5.3.3 with FastCGI on Apache