Thank you @JC for anwering my question.
I did what you advised me to and suceeded in bios recovery.
My NUC is now up and running again.
Strange enough the past days my NUC did shutdown well several times.
I keep an eye on its behavior to be sure all problems are gone now.
What i did not understand is why to remove the upper ram-module. There is room enough for removing the yellow jumper.
I am glad to hear the unit is working as it should now.
Regarding your question about the memory, it was a troubleshooting step just to make sure the issue wasn't being caused by a bad memory stick, no other reason.
Please let me know if you need further assistance.
Unfortunately updating BIOS as advised was no solution.
It looks like updating was a temporary solution.
After BIOS recovery my NUC did no shutdown but did a restart three times, two times my NUC started (after showing its BIOS-screen) with a black screen.
After 19/4/2017 nog other changes were made tot hardware of drivers.
No I didn't try.
That's a day work.
This "sollution" is like a garage telling me to replace the engine when the doors won't close.
The first time of my life i have a Intel computer with a Intel motherboard with a intel processor and Intel drivers. Now the only thing to blame is Microsoft?
This is a troubleshooting step to make sure the issue is related to the hardware or software and shouldn't be seen as a solution.
The reason why I ask to reinstall the OS is because sometimes it gets corrupted, as you informed in the initial post that this didn't happen at the beginning.
You can also contact support to check for any available warranty options.
Warranty Services FAQ
Please keep me updated.
No i did not.
As I told you, reinstalling Windows and all other things is al lot of work.
As a test last week I installed the latest BIOS from Downloads for Intel® NUC Kit NUC6i5SYH
The first boots were excellent but after that the problem came back.
Hopefully, any other peer will input useful information regarding this matter.
In case you don't want to reinstall the OS, I would recommend contacting support to look for any available warranty options.
I hope this helps.