In my case, not only could I not connect AirDisks, but I couldn't open up the configuration in AirPort Utility. In additon, AirPort Utility would show an orange error light, while the physical AirPort Extreme had a green light. I managed to hard reset the Extreme (unplug power, hold reset button down, then plug power back in), but this appeared to be only a temporary fix, and the problem would re-emerge after I re-entered my settings.

The only permutation that I came across that would work was to set the connection to 802.11n 2.4 GHz (it was 802.11n 5 GHz previously). Since then, it works as expected. I haven't tried reverting the firmware. Another post somewhere recommended selecting 802.11n 5 GHz, then option-click the channel to change it to 36. I haven't tried this either yet.