Comment 10 for bug 842194

The file /etc/lightdm/ligtdm.conf is being created while running postinst script from lightdm-{kde,gtk}-greeter package. The script needs the /usr/lib/lightdm/lightdm-set-defaults binary. This binary is owned by lightdm package so it is necessary to install lightdm package before or at the same time with lightdm-{kde,gtk}-greeter. If lightdm package was installed after one of the lightdm-greeter packages, you may run "dpkg-reconfigure lightdm-kde-greeter" or "dpkg-reconfigure lightdm-gtk-greeter" (depending on the greeter package you have installed) to generate the proper /etc/lightdm/ligtdm.conf file. This method helped me to solve the problem.