Error logging out

[Rich Shepard]

Running Xfce4-4.16 on Slackware64-15.0 on a ThinkPad X200. Using
Applications -> Log Out used to bring up a window with options, and
selecting Log out and shut down worked.

This morning I set up Sessions and Startup to mount external USB sticks when
inserted following instructions from a web page dated 2011-10-11.

When I tried to log out from the Applications menu a message box appeared
with the following error message:

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

What might I have done wrong setting up automount and how to I fix this
error?

TIA,

Rich