No Display Manager in Kubuntu 16.04 After Upgrade From Kubuntu 15.10

[Stephen Morris]

Hi,

     After receiving notification that there was an upgrade for Kubuntu 
available (not that I actually have Kubuntu installed, I have Ubuntu 
installed with KDE installed as an addon), I ran a sudo 
do-release-upgrade to undertake the upgrade. This process downloaded 
between 2500 - 2600 packages.

     After the upgrade, when I boot into the os, the boot process wants 
to start SDDM but the start fails, and every re-attempt to start it 
fails. Using systemctl to attempt to start the service manually fails 
with no meaningful messages, and systemctl status sddm.service doesn't 
show any meaningful messages either. If I issue the command, systemctl 
start lightdm.service, that starts the DM quite happily.

     I have checked /etc/X11/default-display-manager and that had the 
command /usr/bin/lightdm as the display manager to launch, but that 
location in not the location where lightdm is installed. I have replaced 
this command with /usr/sbin/lightdm, but on a reboot the boot process 
still tries to unsuccessfully load SDDM. I have also noticed that the 
location of SDDM, unlike Lightdm, is actually /usr/bin/sddm.

     Is anyone able to shed some light on why the 16.04 boot wants to 
always start SDDM even though the default display manager is set to 
lightdm, and also why sddm cannot actually start? Could it be that the 
sddm.service process is looking for sddm in /usr/sbin when it is 
actually in /usr/bin, and if so, would a symlink in /usr/sbin back to 
/usr/bin/sddm rectify the issue?

regards,

Steve