login screen does not start after upgrading to kubuntu 18.04

After an automatic upgrade from ubuntu 17.10 to 18.04 (sudo do-release-upgrade) my login screen is black. The xorg.conf, Xorg.0.log, /var/log/gpu-manager.log and the output of nvidia-bug-report are available here:
I have little or no experience in this field. Is there any other info I can provide to solve this?
Thank you

The following commands solved the problem…

sudo apt-get purge nvidia*
sudo apt-get install xserver-xorg-video-nouveau
sudo dpkg-reconfigure xserver-xorg