Hi Laura,
every AP chached a key if he get connect the first time with his controller.
This should help to boot faster if the AP reboots and can find his controller faster.
This key was cleared somehow between the Controller and the AP.
Maybe the AP was taken from a different Controller where he was connected and also in function..or maybe you got a power outage and this cause this mismatch.
Anyway..the only possibility is to set the AP to factory default ( ssh to the AP) and delete them in the same time on the controller so they can negotiate a new key and then it should be all good.
Regards
Umut
