Comment 4 for bug 828576

I guess problem with starting up lightdm after plymouth is in misconfiguration.

$ cat /etc/init/lightdm.conf

... SKIPPED ...

script
    if [ -n "$UPSTART_EVENTS" ]
    then
 [ ! -f /etc/X11/default-display-manager -o "$(cat /etc/X11/default-display-manager 2>/dev/null)" = "/usr/bin/lightdm" -o "$(cat /etc/X11/default-display-manager 2>/dev/null)" = "/usr/sbin/lightdm" ] || { stop; exit 0; }

 if [ "$RUNLEVEL" = S -o "$RUNLEVEL" = 1 ]
 then
     # Single-user mode
     plymouth quit || :
     exit 0
 fi
    fi

    exec lightdm
end script

... SKIPPED ...

It checks if there is "/usr/bin/lightdm" or "/usr/sbin/lightdm" in /etc/X11/default-display-manager, but in my case it's "lightdm" in that file:

$ cat /etc/X11/default-display-manager
lightdm