Answer by m00am for "The following packages have been kept back:" Why and how do I solve it?

For the very specific flavour of this problem where the proprietary nvidia drivers are halfway updated to a new version, I reinstalled them using the driver manager. To give some context: I was stuck halfway between the 440 and 450 version of the driver and a whole package of libnvidia 440 packages were kept back. This resulted in my kubuntu being stuck at the spash screen after grub. To get into the system, I had to add "nomodeset" to the grub command as described here.

In this specific case

sudo apt-get --with-new-pkgs upgrade

did have no effect. However, I was able to reinstall the drivers through the additional driver management. In my case, on Kubuntu I started

sudo kubuntu-driver-manager

On Ubuntu you can reach the same thing via System Settings > System > Software & Updates > Additional Drivers

There I selected the 450 driver and the graphic drivers were reinstalled, resulting in a properly booting machine.