Font rendering poor

This morning I installed updates for Zorin and since that time the display is poor, fuzzy.
The graphics driver details:
vendor : NVIDIA Corporation
model : GM107M [GeForce GTX 860M]
driver : nvidia-driver-418-server - distro non-free
driver : nvidia-driver-440-server - distro non-free
driver : nvidia-driver-450 - third-party free recommended

That recommended driver is installed.
I tried different fonts but the rendering remains poor.

I have two questions:

  1. how to repair the poor rendering
  2. how to revert to a previous installation without making backups

I just noticed that the repository used has changed back to one that doesn’t allow updating:
The repository ‘ artful-updates Release’ does not have a Release file.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

You might check your DPI.

For the repository:
You could try tremoving the outadted pp with

sudo add-apt-repository --remove (PPA NAME)

then adding the proper ppa

sudo add-apt-repository ppa:graphics-drivers/ppa