I am using Debian/Sid and encountered the same issue. upgrading to lightdm_1.19.2-0ubuntu1_amd64.deb fixed all issues.
- Bug#829557: lightdm 1.18.2-1 breaks D-BUS Tsu Jan
- Bug#829557: [Pkg-xfce-devel] Bug#829557: lightdm 1.... Yves-Alexis Perez
- Bug#829557: [Pkg-xfce-devel] Bug#829557: lightd... Vincent Lefevre
- Bug#829557: lightdm 1.18.2-1 breaks D-BUS Jos van Wolput
