Somehow, while playing around with LightDM and Webkit Greeter, the ownership of the
.Xauthority file in my home dir was given to the root user and I couldn't login because I hadn't the privilegues to lock the file.
I was able to regain ownership of the file and I could login again. (After several hours of reinstalling LightDM and it's greeters)
So now everything's working fine again. But I'd like to know how this happened. Is this a bug in LightDM or Webkit Greeter or something else?