Today I had an issue which allowed me to see the login screen from my user, but I was not able to log in. Whenever I confirmed my password, the screen got black and then the login screen restored without me being logged in.
The reason for that was most likely a file called
.Xauthority which deals with the display side of things, the Xorg display. That file got messed up by probably a package that I had installed (it was a flatpak package called what-ip).
So in order to solve that issue I did remove that file with
Deleting that should in theory reset the bad formatting and I can confirm that it worked. A new
.Xauthority file got created and now I can login into my user again.