So, after crimping both ends again:
RB4011 --PoE--> hAP doesn't work (fast blue blinking, "not enough power")
RB4011 --PoE--> hAP+24V PSU works
RB4011 --PoE--> hAP+24V --PoE--> UAP AC Pro works (passing through 48V from the RB4011 PoE-In)
RB4011 --PoE--> UAP AC Pro suddenly also works with GbE 🤷
I guess I must never touch or reboot either one of them now.