I have the same problem now, with Zorin OS 16 (final release), which is based on Ubuntu 20.1, if I'm not wrong.
I hope in'sha'Allah that I'll find a solution for this, or that these bugs would disappear after the next update. You can try installing
xsrceensaver, as mentioned by benfischer on Launchpad:
sudo apt install xscreensaver
or you can try updating NVIDIA drivers (you can also remove them and reinstall them) as mentioned by System76.
sudo apt purge ~nnvidia
sudo apt autoremove
sudo apt clean
sudo apt update
sudo apt full-upgrade
sudo apt install system76-driver-nvidia
After the installation has been completed, run:
sudo systemctl reboot
Now, try logging in again.