Ok, i'm completely confused...
After trying almost everything, i got the feeling that the problem was caused by the DHCP Server, which was my AP at the same time (DLINK DWL 900+). So i gave myself a try and configured my Linux Router as DHCP Server. And guess what? At the moment everything is working fine
Think i'll wait till tomorow, lets see if it works then.
What is your DHCP server? The AP, too? Then try to seperate the AP and the DHCP Server to 2 different machines. Tell me if it works or not!
Greetings