When <seat>.conf
[Seat:*] type=xlocal
and <greeter>.desktop X-LightDM-Session-Type=mir
The log in /var/log/lightdm is called null. It should be called something like x-greeter.log
When <seat>.conf
[Seat:*]
type=xlocal
and <greeter>.desktop Session- Type=mir
X-LightDM-
The log in /var/log/lightdm is called null. It should be called something like x-greeter.log