If you get the occasional GDM error at startup in Cassandra, Celena or Daryna here is a method that should completely resolve this from happening.
As root, edit your /etc/hosts file by putting the computer name in the top line skipping a space.(An example of this trick is in the WIKI.)
Open the Control Panel and open Sessions. Click on add. I used this for the name,"GDM Daemon". Use this for the command, "gnome-settings-daemon", and last put whatever description you like. I used, "bug fix" for a description so I would remember why I created this.
My computer has been booted at least 60 times without a GDM settings error. This problem comes from the Ubuntu base that LinuxMint is built from.
Hope this was of help!