SUCCESS!!!
Not sure what it was in the end.
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Removed all suggested 3D bloatware - Asus Splendid Video Entertainment, Nvidia Geforce Experience, etc. etc, restarted, removed some more whatever seemed not necessary, NO SUCCESS.
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Removed Rhino 6 including regedit again, restarted, re-installed, NO SUCCESS; updated to the latest version from yesterday (Rhino 6 SR4 2018-4-10 (Rhino 6, 6.4.18100.1591, Git hash:master @ b6485d60ae29d5ee76751120dc9232f8b172e934)
Licence type: Educational, build 2018-04-10) - but then, Rhino 6 crashed at the start in a slightly different way: it remained “responsive” with the first splash screen where one can select a template, so something changed. -
For whatever reason, Windows 10 just found another of these automatic cumulative updates to perform today, after it had been updated continuously before, last updates 9.4.2018. 16.3.2018, etc (I never delayed these) - and NOW … SUCCESS!!!
Now Rhino 6 runs nicely : ) with the Boltgen plugin also installed again (latest one there as before).
Thanks for all the suggestions.
Even if we cannot know what it was, I am sure the answer must be somewhere in the region between Rhino 6 and some video or other system aspect, and some improvements along the line from various sides.