[Milhouse]

@Milhouse thank you gain for your reply. I just installed Ubuntu 18.04.1 LTS on tht NUC . "without" problems. Only problem: I hve to unplug and replug HDMI after initial boot.
fresh install:

When booting the LibreELEC installer, have you tried changing HDMI ports on the NUC (unplugging the cable from HDMI1 and plugging into HDMI2, or vice versa)? I seem to recall an issue with Intel NUCs a while ago where the output port would switch resulting in blank screens (either that, or I've mis-remembered what the problem was at the time).

Can you disable the "other" HDMI/Display Port/whatever output in the BIOS?

Edit: Also, do you have the option to boot using BIOS (Legacy) or only UEFI? In the boot manager you should see the USB boot drive listed twice, once for UEFI and again without UEFI - try both (if you haven't already). As you can tell, I'm running out of ideas.