Do the APs have static IP addresses? If so, is spanning-tree port-fast enabled on the switch ports where the APs connect? There was a change in a timeout value that was put in place around 5.0.3. It only waited 10 seconds for an AP to find it's master. If port-fast was disabled, it took longer than 10 seconds for the port to go active and the AP would reboot before it ever got connectivity.
The way to resolve this issue is to enable port-fast on the AP ports.
The timeout value was changed back to 60 seconds in newer revisions of ArubaOS.