I've experienced the short circuit error on the PowerBox in previous firmware version when I was powering the unit via PoE, but this is something new.
The option to avoid the short-circuit on long cables doesn't exist any more with new software versions. I have a PowerBox that I'm trying to use to power two Ubiquiti 24volt radios (900-M and M5-PowerBeam). If I plug a 24volt PoE source into Port 1, everything comes up properly. If I plug power 24 volt power into the barrel connector the PowerBox comes up but repeatedly says Short-Circuit detected on Port 1 (where nothing is plugged in), and Port 4 and Port 5. The PowerBox is being powered over a 400ft DC cable spliced to the barrel connector.