thnx, but i think I found the cause. This NUC was built to replace a packardBell which was connected to 15meter HDMI cable. its only purpose was to constanty show a webpage.
I think the NUC has trouble with the long HDMI cable. I took the NUC home and rebooted it a couple of times at my test HDMI screen, works just fine.
Is there something like a HDMI cable which is too long? the packard bell computer never had this issue. Or is the NUC too underpowered to handle the long HDMI cable?