I had a problem with Airport Extreme and D-Link 300T. I could only connect one computer at a time to the internet, and if I used the APE to distribute the IP addresses it stopped working altogether.
I reverted the APE firmware from 5.6 back to 5.5.1 and it's worked fine ever since with the APE distributing IP's in the 10.0.0.** series..
I wonder if there was a parallel upgrade to the Airport Express firmware that is causing a similar problem?