suspend fails with "GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: Operation not permitted"

[Juanjo Aparicio]

* System release:
Description: Ubuntu 13.10
Release: 13.10
* Package: unknown
* Expected behaviour: Clicking on the "Suspend" option of the session menu shows a confirmation dialog, then the computer goes to sleep
* Actual behaviour: Clicking on the "Suspend" option of the session menu shows a confirmation dialog, then an error dialog, with the text "GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: Operation not permitted"

Additional information:
Just upgraded from Xubuntu 13.04 to Xubuntu 13.10
Kernel 3.11.0-13-generic
Proprietary nvidia module loaded (nvidia-319-updates=319.60-0ubuntu1, nvidia-common=1:0.2.83, nvidia-settings-319-updates=319.60-0ubuntu1)
Running xubuntu-session
Happens with existing users (existing .config, etc) and with newly created users also.

.xsession-errors shows no additional info

Using dbus-monitor and repeating the sequence of actions shows, among others, the following lines:

method call sender=:1.50 -> dest=org.xfce.SessionManager serial=95 path=/org/xfce/SessionManager; interface=org.xfce.Session.Manager; member=Suspend
method call sender=:1.5 -> dest=org.xfce.Xfconf serial=153 path=/org/xfce/Xfconf; interface=org.xfce.Xfconf; member=GetProperty
   string "xfce4-session"
   string "/shutdown/LockScreen"
error sender=:1.8 -> dest=:1.5 error_name=org.xfce.Xfconf.Error.PropertyNotFound reply_serial=153
   string "Property "/shutdown/LockScreen" does not exist on channel "xfce4-session""
error sender=:1.5 -> dest=:1.50 error_name=org.freedesktop.DBus.GLib.UnmappedError.GDbusErrorQuark.Code9 reply_serial=95
   string "GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: Operation not permitted"

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[dsiroky (siroky)]

Mixture of solutions: http://www.smallbulb.net/2014/640-xubuntu-upgrade-to-13-10-with-troubles

[Si Dedman (si-dedman)]

@dsiroky did you get any of those to work? in my case the problem is SLIGHTLY different, i.e. I have:
"Power manager
GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: Permission denied"

whereas he has
"Failed to hibernate session
GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: Operation not permitted"