Usually, when the nvidia driver does not work, nouveau comes to help. But this time, with 15.2, it was the opposite.

I installed Plasma, with graphics card from Nvidia on GK208 chip (a small and weak graphics card, but I don't require much of it).

Nouveau caused severe crashes of graphical system (windows, cursor, keyboard stopped working, but disks, network and sound card continued running). There were no traces left in log files. Crashes were happening every day and required rebooting.

It took me days, much work and anger to figure out the reason for crashes, although I had it in my mind that graphics driver might be reason. I hoped that OS updates would fix the crashes, but they didn't.

Installation of nvidia driver solved the problem.