Comment 6 for bug 1855350

I don't have much to add other than to say I am also experiencing this issue or one similar to it.
I am running xfce on ubuntu 20.04 in a kvm virtual machine. The system was configured to boot to a command prompt with “systemctl set-default multi-user.target”, so when I want to run xfce this is done with "startxfce4".

(problem description)
if you log in to xfce then immediately log out it will fail with one of these errors:
session manager must be in idle state when requesting a shutdown
gdbus.error:org.freedesktop.dbus.error.invalidargs: type of message, "(yb)", does not match expected type "(b)"
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example scenarios:
reach the desktop, wait 45 seconds or more, try to log out, it works normally
reach the desktop, try to log out immediately, get an error, try a second time, sometimes it will work
reach the desktop, try to log out immediately, get an error, sit there doing nothing for a few seconds, sometimes it will log out
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I tried playing around with the file 50-guest-wrapper.conf as others had mentioned, but I did not notice any difference.
I also have an identically-configured centos 8 vm and it does not have this issue.

(uninformed guess)
When reaching the desktop something related to lightdm or xfce is failing in the background, once a timeout is reached, things work as expected.