Hi,
Yes, it was apparently an issue with my p2 bundle pool used by Eclipse Installer. When checking the error log in more detail, it revealed lots of bundle exceptions related to unresolved modules.
So after removing all my different installations based on this shared bundle pool, and then removing the p2 bundle pool itself and starting all over reinstalling everything again, then the issue is no longer there.
The major confusion was probably that the issue first appeared in my Papyrus Oxygen installation yesterday (hence my heads up for M6 spin), which then was fixed when I updated my Papyrus Oxygen installation earlier today. So I assumed that the work yesterday with the M6 had made some change that actually fixed the issue I could see.
And then when I updated my Papyrus-RT installation, which is based on the latest Papyrus Neon nightly build, and it failed with the same symptom as for Papyrus Oxygen I had yesterday, it just felt extremely strange. I then checked my separate Papyrus Neon installation, updated it to the latest build, and it then had the same issue.
Anyway, after reinstalling everything, including removing the p2 bundle pool, it now works again. Sorry for raising this personal issue of mine, and any confusion it might have caused.
/Peter Cigéhn