Login at the moderator panel not working
 

I have a testuser account with which I wanted to check the permissions that my moderators have in the mod panel.
So I changed the testuser's usergroup to moderators (the usergroup that all my mods are in).
I then tried to log in at the moderator control panel. It showed me the redirect welcome message, but I am redirected to the login again!
I already deleted the cookies, but with no avail.