Comment 14 for bug 1431654

FYI, in case there are some lesser experienced users that have this issue and are looking for a temporary solution for the time being, below are the instructions to revert to the previous release of lightdm until this gets resolved.

You can do this with the following command in a terminal (<CTRL + ALT + T>):

    sudo apt-get install lightdm=1.2.3-0ubuntu2.5

Then to kill the current lightdm session and start a new one using the previous released version, log out, then press <CTRL + ALT + F1>. Log into the fullscreen terminal session. Then type the following:

    sudo service lightdm stop

    sudo service lightdm start

Then to close the terminal session, press <CTRL + ALT + F1> once again, type exit, then <CTRL + ALT + F7> to return to your lightdm session. After that, you can log in and you're back to normal.

I also locked the lightdm package after reverting back for the time being so that I don't accidentally reinstall the broken version by doing the following in a terminal:

    sudo echo "lightdm hold" | dpkg --set-selections

Once this gets fixed or a new version is released later on, you can release the lock/hold with the following command in a terminal:

    sudo echo "lightdm install" | dpkg --set-selections

Hope that helps!