Thank you for your suggestions, Willie & Karl
My Windows 8.1 still receives Updates (have the Pro version) and extended support will last until January 2023.
Disabling the Windows Update Service in Windows 10 on the one hand blocks unwanted major upgrades. But on the other hand it also prevents security and anti virus Defender updates which I still get for Windows 8.1
In two years, I admit, things will look different.
It's Windows 8.1 64 bits, otherwise the memory upgrade to 16 GBytes RAM wouldn't have made any sense.
Forgot to say that in terms of IT I'm a kind of pro and software developer, too. Have been managing servers, network and our own software for aircraft deicing and towing at the Munich Airport (and some others) for years.
So a motherboard change is not that big challenge ;-)
And if it should fail - I have a backup I made right before.
In the device manager all virtual COM ports showing as "Eltima Virtual COM ports" are present and working perfectly. The drivers are the latest version from the Eltima web site for Windows 8.1 and 10 and fit the hardware used in my MicroHam USB Interfaces III.
The problem is: All software accessing the COM port/rigs directly is working perfectly, i.e. HRD making a connection to my TS-950SDX via COM5.
And WSJT-X using the direct hardware connection via COM5 is working, too.
Also other software as HDSDR accessing the rig via OmniRig is working, too.
Only WSJT-X trying to to connect via OmniRig is causing problems.
Klaus / DF6NP