Second thought: I do have a 2017 version of Nikon Nef Codec installed, so may be that is what is used for the raw conversion? In that case it should give results very close to the Nikon software.
Thanks.
Installed the Codec on W7 and W10 machine, restarted and it did recognize the NEF's. The W10 works also very fast. I guess with the W7 there are more problems.