Hello,
when I try to log out from desktop (Start->Log Out) I'm getting a message box error with "Session Manager expects system shutdown. Please wait" text. I've tried to investigate source code of xfce4-session-logout and found that g_dbus_proxy_new_for_bus_sync returns NULL value instead of GDBusProxy:
xfce4-session/xfce4-session-logout/main.c:
.....
GDBusProxy      *proxy = NULL;
.....

proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SESSION,
                                         G_DBUS_PROXY_FLAGS_NONE,
                                         NULL,
                                         "org.xfce.SessionManager",
                                         "/org/xfce/SessionManager",
                                         "org.xfce.Session.Manager",
                                         NULL,
                                         &err);
  if (proxy == NULL)
    {
      xfce_session_logout_notify_error (_("Received error while trying to log out"), err, have_display); /*seems this i'm getting*/
      g_error_free (err);
      return EXIT_FAILURE;
    }

Is any way to manually debug this situation? If so how I can do it? And what datas shoud I provide for you to investigate this issue?
My XFCE version is:

xfce4-session 4.12.1 (Xfce 4.12)

Copyright (c) 2004-2012
	The Xfce development team. All rights reserved.

Written by Benedikt Meurer <benny@xfce.org>
and Brian Tarricone <kelnos@xfce.org>.

Please report bugs to <http://bugs.xfce.org/>.

WBR,
Alex