Game closes/crashes? when in friend enters the room

when my friend joins in a DUEL match our games/clients close out.... how come?

i'm Using 2008 server and client files

Ok so i realised that it isn't just for the duel matches, it's on any game type, once my friend joins in the game both of our clients just shutoff, why is this, does anyone know? :]

OK, even MORE info =]
i had made him administrator and upped his level to 99 with a bunch of exp and the game only crashes when he uses that 1 character... he created another character and the game didnt' crash