Game and Steam crash when joining

[von]

When I try to join a game my game freezes and when someone tries to join my game their game freezes.
I can't see any servers when I'm browsing the serverlist too.
Does anyone know what the problem is?

EDIT: It's probably my Antivirusprogram. It's since the 2015 version came out it's blocking every f***ng program.

[WitchedDoctor]

I've been crashing everytime i look at the server browser sucks.