Here are the facts:
I ran some stress-tests last night. I forgot to reset the logs to minimal. When I got this morning, I saw that hard drive is full /dev/sda1. Cleaned up the large file (350GB). I experienced the machine being a bit slowlier than usual. I rebooted the machine.
Since then, when I try to login, the computer shows as if it let me in, and yet, prompt for login again.
If I provide the wrong password on purpose, it shows the usual error. When I provide the right one, it is like an infinite loop.
When I hit Ctrl+Alt+F1 and login at tty1, for instance, all works fine.
From tty1, I tried to setup auto-login as follows:
sudo vim /etc/lightdm/lightdm.conf autologin-user=<username> autologin-user-timeout=0
Yet, it doesn't perform the auto login for some reason.