Got the problem, found the answer :
- Log into your account using a terminal, which you can reach using Ctrl + Alt + F2.
- Look at the permissions for your
.Xauthority file, using
ls -la ~/.Xauthority.
It needs to be readable and writeable by your user :
-rw------- 1 you yourgroup .Xauthority
(usually, group and user are the same for regular users)
If your permissions are invalid, you can apply a fix by doing :
chown you:yourgroup ~/.Xauthority
chmod 600 ~/.Xauthority
Then go back to your display manager with Ctrl + Alt + F7, and try again !
If it still fails, have a look a your X11 logs, and share :
cat /var/log/Xorg.log.0 | tail -n50