Ubuntu – How to solve “Stopping User Manager for UID 121” error after installing Nvidia drivers on Ubuntu

Here's a slightly simpler solution: I found that using lightdm the nomodeset parameter was the key to fixing my issue with 18.04

Nomodeset: How to get in once via temporary grub change

You can add the grub configuration parameters if you hold right shift at bootup and press e to edit Ubuntu parameters. You need to replace quiet splash with nomodeset. Then hit ctrl x to save and continue.

Lightdm: How to put in the permanent fix

Once in, open a terminal and type sudo apt install lightdm, then sudo dpkg-reconfigure lightdm, then sudo reboot

Please refer to this amazing guide for more details and screenshots!:

https://askubuntu.com/questions/162075/my-computer-boots-to-a-black-screen-what-options-do-i-have-to-fix-it

Generic way

If you'd like to have the recommended packages installed too, then you could run this (the version was last updated on 2020-Dec-22):

sudo apt-get install --install-recommends nvidia-driver-455

I think I will not update the version anymore, so I will tell you instead, how to find out (manually) that there is a new version.

As there are many ways, the most comfortable for me is (as a normal user or root) typing to terminal:

apt-cache policy nvidia-driver-4

and double-tapping the Tab, an example output follows:

nvidia-driver-418         nvidia-driver-435         nvidia-driver-450
nvidia-driver-418-server  nvidia-driver-440         nvidia-driver-450-server
nvidia-driver-430         nvidia-driver-440-server  nvidia-driver-455

The resulting status in Linux Mint specifically should be afterward:

Linux Mint 20.0 - Driver Manager

Ubuntu way

Thanks to the Ubuntu base, one can also take advantage of, which takes care of everything, but I do not recommend it due to one has no control over what happens, and things can break as a side effect, so the following I note only for completeness:

sudo ubuntu-drivers autoinstall

To only list drivers applicable to your system, you can do:

sudo ubuntu-drivers list

which I did not know there is, even (I came across it here).