I did something a while ago, and I can't remember what it was, but it broke LightDM. My solution since has just been to use GDM, but since I like unity-greeter more, I'd like to get this sorted out once and for all.
The problem is that LightDM loads fine and allows me to type my password in etc, but once I press enter or click 'log in', the screen simply flickers and reloads LightDM again. This happens with any account, including guest.
As I implied, GDM works fine. I've also tried reinstalling every LightDM-related package and also apt-get purge. It does nothing to help - I even tried deleting the entire lightdm config folder (where unity-greeter is) and the problem persists.
Any help would be appreciated :)