I am sorry you are having this problems but let me help you.
Checking the website I saw there is new BIOS that fixes some USB issues with this NUC. Please try this update and let me know.
Here is it:
Thanks for your answer.
I saw yesterday these new versions (specially V1.69) and thus downloaded V1.70 to my board.
Now problem is sometimes boot is ok, sometimes I encounter the same problem.
I am now testing with USB legacy disable since (if I did not misunderstood) it should desactivate all USB communication during Boot process... Is this a good idea?