Looks like this isn't getting much attention however I have the same issue in a number of hours on the WAP121 and WAP321. It is clockwork and a reboot "resolves" it for the nest little while. This is not an acceptable solution.
Chatty devices can make it happen very quick like a Windows Phone 8 whick tries ESP-MSCHAPv2 a number of times before EAP-PEAP-MSCHAPv2 for instance eventually causing the accesspoint to stop sending any RADIUS attempts at all no matter what device attempts to connect until the accesspoint is rebooted again. The server logs will begin showing challenges again after the AP reboot.
This is the most critical function of these devices and they are failing at it.