Suspend (to RAM) has been working on my machine for years, but it broke
a couple of months ago. When I try the keyboard shortcut I've configured
(that has worked for years), I get an error popup window:

Received error while trying to log out
GDBus.Error:org.freedesktop.DBus.Error.InvalidArgs:
Type of message, "(yb)", does not match expected type "(b")

(I've googled this message and found other references to it, but no
clear solution / interpretation:

https://unix.stackexchange.com/questions/626284/thunar-bug-should-i-report-to-xfce-or-ubuntu
https://askubuntu.com/questions/1233288/unable-to-logout-from-guest-session-on-xubuntu-20-04
https://bugzilla.redhat.com/show_bug.cgi?id=1729059
https://forums.freebsd.org/threads/xfce-wont-let-me-log-out-anymore.76968/
https://www.reddit.com/r/xfce/comments/bej4xx/received_error_while_trying_to_log_out/ )

When I try the little "sleepy moon" icon, I get a different popup window:

Failed to run action "Suspend"
GDBus.Error:org.xfce.SessionManager.Error.Failed: Failed to lock the screen.

Suspending via "systemctl suspend" works fine.

I should have reported that the command bound to the keyboard shortcut
is "xfce4-session-logout --suspend". When run from the command line, I
get a popup window with the message

"Received error while trying to log out
GDBus.Error:org.freedesktop.DBus.Error.InvalidArgs: Type of
message, "(yb)", does not match expected type "(b")""

and the command line message:

"Received error while trying to log out, error was
GDBus.Error:org.xfce.SessionManager.Error.Failed: Failed to lock the
screen."

Celejar