If you had the issue already that basically everything in your KDE environment stops working all the sudden after a dist-upgrade and you end up on a black screen instead of the plasmashell after login, then you can try this.
It solved this issue for me.
Add this line into your .bashrc
At the next login the desktop should appear again.
The problem seems to be that older nVidia cards have some buggy execution of some of the more modern OpenGL 3.1 commands, so forcing it to be using OpenGL 2.1 is safer.