Always check if the AP has all of the necessary firmwares in it’s storage.
In our case we received the following error messages:
Sep 19 06:49:38.214: %LINEPROTO-5-UPDOWN: Line protocol on Interface Dot11Radio0, changed state to down Sep 19 06:49:40.426: Interface Dot11Radio0: not starting because stop-on-failure set Sep 19 06:49:40.430: %LINK-6-UPDOWN: Interface Dot11Radio0, changed state to up
The antenna stayed in up/down or init/down state.
After copying the right firmwares and restarting the device the problem solved.