I had this same issue today. RB260GSP (FW v1.14) was powered over ~20 meters of 1 sq.mm wire via DC plug with 27.6V/2.4A PSU and threw "Short circuit" in PoE status no matter what settings were applied or what device connected. LEDs on RB I was trying to power-up kept blinking shortly with onboard speaker "ticking". Then I brought RB260GSP right to the PSU (eliminating the 20m power cable) and everything worked fine there. Checking "Port1 PoE in Long Cable" in System tab helped to resolve this problem, though power was supplied via DC port only
This definitely needs some more documentation or mentioning in forums. The wiki explanation of what that Long cable setting does is also a bit sketchy.
The RB260GSP is a great product, but in my opinion needs a little more firmware refinement. For example PoE status messages are unclear or even misleading. Seeing "Short circuit" message we checked cables for shorts, tried different devices, etc. I don't know how powering circuitry on these switches work but it never works correctly out of the box for me. We shall try to connect some higher capacity capacitor in parallel tomorrow to absorb power spikes during device startups and see if it helps (without Port1 long cable setting).