lightdm cannot be started manually when it's not default DM

[Serhiy]

ExecStartPre=/bin/sh -c [ "$(basename $(cat /etc/X11/default-display-manager 2>/dev/null))" = "lightdm" ] (code=exited, status=1/FAILURE)

Why would you need such a check? It was just fine starting DMs manually previously without making them default.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: lightdm 1.26.0-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-34.37-generic 4.15.18
Uname: Linux 4.15.0-34-generic x86_64
NonfreeKernelModules: wl nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.3
Architecture: amd64
CurrentDesktop: KDE
Date: Sun Sep 9 16:19:30 2018
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)
upstart.lightdm.override: Error: [Errno 13] Відмовлено у доступі: '/etc/init/lightdm.override'