I was having the same issue. I discovered the issue when I found errors
written to `/var/log/seat0-greeter.log`:

    Error writing X authority: Failed to open X authority
/var/lib/lightdm/.Xauthority: No such file or directory

Indeed, I only had `/var/lib/lightdm-data`. Running the command

    mkdir /var/lib/lightdm

and rebooting fixed the issue for me.

You received this bug notification because you are a member of Desktop
Packages, which is subscribed to lightdm in Ubuntu.

  Lightdm refuses to work after installing kubuntu-desktop

Status in lightdm package in Ubuntu:

Bug description:
  After installing kubuntu-desktop meta package I saw a prompt telling me that 
both sddm and lightdm are available and I am forced to choose one between 
them.so I chose lightdm.on the next reboot I was unable to use GUI.
  After 2-3 weeks,I retried using sudo dpkg-reconfigure lightdm and selected 
lightdm.still fails to run GUI.
  sddm works fine and is able to handle both Plasma and Unity.
  Ubuntu 15.04 with lightdm 1.14.2-0ubuntu1

  ProblemType: Bug
  DistroRelease: Ubuntu 15.04
  Package: lightdm 1.14.2-0ubuntu1
  ProcVersionSignature: Ubuntu 3.19.0-22.22-generic 3.19.8-ckt1
  Uname: Linux 3.19.0-22-generic x86_64
  NonfreeKernelModules: nvidia
  ApportVersion: 2.17.2-0ubuntu1.1
  Architecture: amd64
  CurrentDesktop: KDE
  Date: Thu Jul  9 21:15:22 2015
  InstallationDate: Installed on 2015-06-09 (30 days ago)
  InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
  SourcePackage: lightdm
  UpgradeStatus: No upgrade log present (probably fresh install)

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