“THE SYSTEM IS RUNNIG IN LOW GRAPHIC MODE” erorr is a very common error for LightDM.
When you get this error, click the OK button in the graphical window and tick the box saying “run in low graphics mode for just one session”.
Next, you will have to do this:
$ sudo apt-get install --reinstall unity-greeter
$ sudo dpkg-reconfigure lightdm
Now open the /etc/lightdm/lightdm.conf (with sudo) and make the greeter-session= line look like this:
greeter-session=unity-greeter
This error fixing works for Ubuntu, Linut Mint and other Linux boxes that use lightdm.