It may appear to be an Xorg or lightdm/gdm/mdm error but in reality for many users with this issue, it's a driver conflict and issue. I had a system that had two GPUs, an Intel and Nvidia GPU.
The only thing that got it working was to remove the nouveau driver and blacklist it so it never came back, then the Intel GPU works fine without these issues.
Solution
sudo rmmod nouveau
add nouveau/other driver to blacklist
edit th........
The display manager is more so what controls the main graphical login process after Debian/Mint/Ubuntu boot and controls the graphical login sequence. Once you login, you are then usually passed to an Xorg based Window manager like XFCE, Mate, Ubuntu etc...
Popular display managers are mdm, gdm, lightdm etc... and they all basically do the same thing with a different interface/style and some feature differences.
In Mint for example the normal default display manager is l........
sudo vi /etc/lightdm/lightdm.conf.d/70-linuxmint.conf
Change this:
[SeatDefaults]
user-session=mate
allow-guest=false
To this:
[SeatDefaults]
user-session=mate
allow-guest=false
greeter-hide-users=true
greeter-show-manual-login=true
To see and apply your changes just restart light........