I finally found the origin of all the problems on my dekstop (black screen because windows explorer not starting, unreliable network connections, MS store not working, windows update hanging and more) .... it seems it was a buggy motherboard.
Got hint from a pro about people having problems with motherboards from that mainstream supplier - I used/bought for years for their quality.
Replacing the MB with a competitors one with the same chipset keeping all the other hardware and the problems seem to have vanished (I obviously also uninstalled all the previous MB's apps and drivers).
I prefer not to mention the manufacturers name as this is a public forum.