I found later on my own entry about this topic that I had already with UM 18.04.
The problem is not that I hadn't installed a driver, but being in that part of Software and Updates obviously causes trouble - maybe if we have two graphic cards (NVidia and onboard card). Once I change to Xorg, the system doesn't recognise the NVidia card, and removes all drivers.
I quickly installed UM 20.04 for the 4th time 
and I have now finished the final touches.