Hey guys, can I use PoE out (ether5) of my RB951ui to power up RBSXT5nDr2?

Yes.

OK I will plug it in now. I'm suspecting PoE injector lately because of port-flap issues (and rx fragments, fcs errors). I changed cable, SXT device, now I must try PoE out to rule out the Poe injector.
Port-flapping happened a lot in AC2 and very seldom in RB9521, but both had rx fragments, fcs errors counters increased enormously.

edit: OK it worked thanks! Nothing is burning. Lets wait whether I see port-flapping...
edit2: So far so good! No rx fragments, fcs errors count, no port-flap. It it goes like that, I cant believe that I didnt test the PoE injector before and had my cable + SXT changed, and all those times troubleshooting, sigh.