I have downloaded both Ubuntu and Xubuntu (for sanity check) and write the ISO to USB via Etcher on Mac and Rufus on Windows 7 (again to double check).
I have installed the latest reFInd and I have tried to boot the USB stick (the USB stick was used to boot other HP server etc.. so its not broken) in every combination I could think.
reFInd UFI boot, reFInd legacy boot, native OSX boot (via holding ALT key).
All of them hang here:
OS X 10.11 ("El Capitan") System Integrity Protection (SIP) feature, aka "rootless" is disabled!