Schau mal hier:
Steampowered FAQ ResultIch glaube immer noch das es an den Ports liegt.
If your configuration software requires port numbers, please use the following:
UDP 1200 (used for friends service)
UDP 27000 to 27015 inclusive
TCP 27020 to 27039 inclusive
TCP 27040 and 27041 only for CyberCafe Owners