lightdm starts neither on startup nor via tty

After a regular upgrade on Ubuntu 11.10 Unity 2D session, I installed gnome-session-fallback package, then removed it (#apt-get purge gnome-session-fallback). This steps drive me to a system in which lightdm does not start. I can try to start it via tty1[2...6] but it did not start.

What I expected to happen is that lightdm starts and greets me.

What happened instead is that it does not start.

Procedure to repeat:

1- Install gnome-session-fallback package
2- Remove completely gnome-session-fallback package
3- Restart computer

PS: English is not my native language so it is possible for me to make mistakes.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: lightdm 1.0.6-0ubuntu1.2
ProcVersionSignature: Ubuntu 3.0.0-14.23-generic 3.0.9
Uname: Linux 3.0.0-14-generic i686
ApportVersion: 1.23-0ubuntu4
Architecture: i386
CheckboxSubmission: 670355b19452e48dc13bba8540f6da92
CheckboxSystem: 331fbefb4b1f6727f4a8261fee7507c9
Date: Sun Dec 25 23:20:24 2011
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Beta i386 (20110921.2)
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)