as you already wrote in your intro, we also found that the resolver is not working properly:ping google.de NOT OK, but ping 8.8.8.8 is okchanging the DNS server for the WLAN NIC to 8.8.8.8 could 'solve' the problem.No other troubleshooting steps were...

You are describing the exact same problem I have! Did you make any progress on the troubleshooting you may want to share? I'm pretty sure that this is a OS problem (Win7) because all users use the same config but only a few have the problems describe...