Both files (/var/log/lightdm.log and /var/log/seat0-greeter.log) have nothing in them. Btw, I have blocked systemd 239, so logout works in Unity.
I don’t have metacity session or gdm in my Unity system. Maybe, the answer to the logout problem of systemd 240 with unity is in the /usr/bin/unity file. For example in Openbox, it is just
openbox --exit the system logs out to whatever login Openbox system has.
My default Ubuntu install doesn’t have gdm, but lightdm and Sleek greeter. It has systemd 240 and logout of the system and opens the lightdm screen. So, the problem with Unity not logging out of the system correctly with systemd 240 is with Unity not logging out of the system correctly. Systemd 240 is stopping Unity from logging out somehow. Something that had been added to systemd 240 that systemd 239 didn’t have, which doesn’t agree with Unity.
Reason: 2 systems – Openbox and default Ubuntu– work well with systemd 240, whereas Unity doesn’t. All 3 systems run lightdm. (I have a quite unique default Ubuntu installation.)
Logout in the drop down panel leads to some systemctl action, which doesn’t work in Unity with systemd 240. I don’t know which one.