I'm running slack8.1, and it's working fine for me (I had a problem with a typo first try though - SASU instead of SaSu
Also, running logoutd made no difference. The user couldn't log in even when it wasn't running.
Just to be sure, start again, with a new user etc, and setting the allowed time to SaSu0000-2400
and if that fails... is your system time correct?
P.S. Don't use su time to login, it won't fail.