The co-op is completely broken! The map in coop gives you no gun and you have to down yourself to get a gun, which btw is empty.  When we restarted the map the host(me) spawned in with a gun, but my partner got nothing and the zombies didn't even chase him. He also could not buy things and was invisible to me. Please fix the co-op.

Me too got this problem in co-op. Singleplayer is perfect though.
Using T4M r41 from here, the current latest T4M has the Server authentification failure problem in co-op mode.

I hope this can be fixed.