I've encountered a problem when my HDD was full in such a way that I could not login anymore. After pressing enter at the login-screen, there was a black screen for about a second and then the login-screen again. It turned out, after quite a lot of searching, that was due to my HDD, which was full. This is something that can easily be checked at every login and which may help quite a lot in debugging such errors. So, I suggest, if the hard disk has less than, let's say, 2MB of free space whilst at the logon-screen, there should be an error-message saying this.
Are you using kdm or sddm as the login manager?
I'm using kdm as login-manager.
kdm is no longer maintained, so this check will not be added. *** This bug has been marked as a duplicate of bug 81653 ***