Display problems after upgrading to 20.04 from 18.04

After upgrade to ubuntu 20.04 LTS from ubuntu 18.04 but I encountered several problems.
After upgrading the whole system didn’t restart but immediately enered rescue mode saying that some file_filter-etc. couldnt be installed.
My remedy was to start from a rescue USB stick and do a boot-repair. Tha at least repiared grub. However not all was well as I am used to work with 2 monitors and the system starts only with one monitor and says “Monitor unknown”. This I detect with the commad Displays from: System _> hardware -> Displays. When I click “Detectmonitors” nothing happens.
When I start Nvidia Settings A blank screen is displayed. Apparently something is wrong with th X-setup but until now I haven’t found what. My videocontroller is the GeForce GT610 and the nvidia driver installed is NVIDIA driver metapackage from nvidia-driver-390 (standard installed) but other offered drivers gave the same results.
I hope somebody can direct me to the solution for this problem. The rest of the system works rather well.