Linux Mint fails to start X automatically after update, but X still works

In some occasions Infinality font rendering can cause problems when loading the Display Manager.

http://forums.linuxmint.com/viewtopic.php?f=208&t=136307

If you have infinality installed and reconfiguring mdm doesn't works, try to:

sudo apt-get remove fontconfig-infinality

you can also reconfigure mdm as follows:

sudo dpkg-reconfigure mdm

this way it won't be necessary to install mdm again, task that may become hard

sudo mv /etc/profile.d/infinality-settings.sh /etc/infinality-settings.sh
sudo chmod a+rx /etc/infinality-settings.sh

To .bashrc, add:

# INFINALITY FONT RENDERING CONFIG
. /etc/infinality-settings.sh

Fixed this issue with:

apt-get remove mdm
apt-get install mdm