Here's something odd I've found with using Ubiquiti passive PoE injectors to power RB1100AHx2: with a newer-style injector [with the white LED] you can power up the unit and run it fine. You can move to internal PSU fine [ie, apply power to internal PSU, remove power from injector/port 13]. However, when you try to move back to the PoE after running on internal PSU, the unit reboots! One way you can tell it's not going to work when you reconnect the injector, the LED on the injector pulses about once a second, and the fans don't speed up [I assume fan speed is linked to voltage - injector voltage is 22.9v, internal PSU is 12.2v].
The older models of Ubiquiti injector [with the green LED, and no reset pinhole on the back] work fine. Fail back and forth 20 times with no issue. Input voltage shows as 22.4v.
So I guess someone's going to tell me, the moral of the story is don't use use Ubiquiti PoE injectors