Update:
I was able to boot LibreElec a couple of times but not in a standard way....
During testing I mistakenly left TWO copies in my Minix, one in the MMC adapter and one in a USB adapter. After attaching power the uSD adapter started flashing and LibreElec was booted after a while. Hoorah?
Current situation:
Until now I was able to boot LibreElec a couple of times, but I do not always have a screen on my monitor and it is not always booting. I always need to have a copy in both the MMC slot and in the USB-adapter.
When booted the display goes blank (some kind of background color to the whole screen) after a while. This can take one minute or 10 minutes or longer, but it happens sooner or later. After that I need to cut the power and reboot.
I will do further testing but if someone can explain why I need to boot this way, please let me know.