I have now 3 instances where either a omnitik-up or a rb750UPA is used to power some radio’s (usually 1, 2 or 3 SXT units) and where either the power out started flapping on and off… or a winbox sessing to the box made it crash… Setting ‘ether1-poe-in-long-cable=yes’ mode made it work.
But the cable length either between batteries (26,5V) or towards the SXT is no longer than 2 meters!
In two instances I have now had a omntik powering 2 SXT units with only a 1mtr cable.
The omnitiks themselves get their 26v battery power over a 10-12mtr cable. Power to the units is fine, but logging in with winbox makes the omnitik reboot…
I can login with a telnet session and have no problem…
After setting ‘ether1-poe-in-long-cable=yes’, no more problems…
In one case I had the omnitik working in this setup (powering 2 SXT-HP units on 50cm’s shielded utp cable) and powered by the standard 220V adapter with an output of 23,6 volts. (measured in the omnitik) for 4 months… Never had any issue.
Yesterday I finally replaced the power adapter for a 24V ups/battery setup and suddenly found the omnitik rebooting itself every 3-4 hours…
So in the evening I started debugging why this is happening, but the moment I login with winbox the omnitik inmediately rebooted (and thus all attached SXT’s; including the BH unit).
I can login from adjacent unit by mac telnet without problem. I checked all settings which are pretty basic and couldn’t find anything.
But it happened again and again and again… 
So I decided to try the cable mode setting mode in the PoE. Set it to ‘yes’ and inmediately winbox could be used. It never went down anymore during the writing of this message (30mins?)
But now my worry is that the PoE-out might not detect short-cuts? How seriously is this? (Before with ordinary power adapter, or battery power, a shortcut just mend smoke and cursing and replacing a unit… 
)
Anybody with the same experiances? Any solutions or advices?
What is ‘long-cable’ versus ‘short’ in this respect? 50cm’s I wouldn’t call “long”…
P.S.
Units all running ros6.7 with updated firmware 3.10