Thank you for joining the Intel communities.
Just checking - You disconnect the power adapter and reconnect it to start the unit, correct?
Please try the following:
Press F2 when you get the prompts to access the BIOS
Once your there press F9 to set the BIOS to default settings and then press F10 to save the changes, your system will restart and check to see if you problem is solved, if not try doing a BIOS recovery.
You can download the same BIOS version 0050 to do the BIOS recovery.
Please see Intel® NUC BIOS Recovery Update Instructions
If that does not work, do you have by any chance a power adapter that you can try or borrow one?
yes, I have to disconnect the power adapter either from the power socket or from the computer for a short moment, to be able to start the computer by pressing the power button afterwards.
Following your advice, I reset the BIOS with F9 to its defaults, but the problem still remains.
However I found out that if I boot Linux from a USB Stick and then shutdown the NUC with "shutdown -P 0", afterwards the NUC can be switch on without any problems and without disconnecting power first. So the whole thing seems to be a problem of my Windows 10 System.
As far as I know there was no Windows system update at the time befrore the problems started. However I installed the program "Redshift 8", an astronomy software which includes Apple Quicktime, just prior to the issue's emergence. I think this installation could have caused my troubles. Mayby it can by fixed by a change in the Windows registry or a setting in the Bios? I'll be offline for a week. When I am back, I could try to deinstall this software and see if it fixes the problem.
Thanks for the information and I hope you can have better experience with the unit from now on.
Please let me know how it goes after you try reinstalling the software.