WiFi shield - Connected, but not able to communicate

Hey everyone,
I'm having trouble getting my Arduino WiFi Shield to work.
I just mounted it on top of my Arduino and then used the sample sketch "WiFiChatServer". The serial monitor shows that the Arduino connects succesfully to my Network. When I now try to connect with Windows' TelNet Client I get an error message, not able to connect.

I really tried everything I can think of - It is not a firewall problem, I can connect my PC to another one in the same Network - Other sample sketches do not work either - my router shows the connected Arduino.

I appreciate any help,
Patrik.