Several panel items do not appear on the panel

Binary package hint: xfce4-goodies

After upgrading to 7.10 I noticed that my system monitor and network monitor panel items had disappeared.

Right click on the panel and select "Add New Item" from the menu.

Add "System Load Monitor".

The item does not appear on the panel.

Running xfce4-panel from the command line yields this output when adding one of the offending items.

The program 'xfce4-systemload-plugin' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadAlloc (insufficient resources for operation)'.
  (Details: serial 185 error_code 11 request_code 53 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)
** Message: System Load Monitor: screen changed: 0

** Message: No valid plug window.

(xfce4-panel:1472): Gtk-CRITICAL **: gtk_socket_get_id: assertion `GTK_WIDGET_ANCHORED (socket)' failed

** (xfce4-panel:1472): CRITICAL **: An item was unexpectedly removed: "System Load Monitor".
The program 'xfce4-systemload-plugin' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadAlloc (insufficient resources for operation)'.
  (Details: serial 185 error_code 11 request_code 53 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)
** Message: System Load Monitor: screen changed: 0

** Message: No valid plug window.

(xfce4-panel:1472): Gtk-CRITICAL **: gtk_socket_get_id: assertion `GTK_WIDGET_ANCHORED (socket)' failed

** (xfce4-panel:1472): CRITICAL **: An item was unexpectedly removed: "System Load Monitor".

The panel items that suffer from this problem are,

Battery Monitor
Network Monitor
Volume Control