I'm using the proprietary nvidia-313 driver for my GTX 460.
Ever since I had some Harddrive problems and had to recreate the xconfig, when I want to start Skype or dmenu, LightDM seems to crash, because the next thing that happens is that I am back at the login screen.
I already tried to reinstall LightDM and to rewrite the xconfig via the nvidia driver. Nothing helped so far. I've no idea, why just these particular programs are affected. Neither a terminal window, nor firefox or thunderbird or whatever else is affected.
I tried to google this, but I had no luck there either.