clean installed system, new user and default keymap (us)
I run "xfce4-panel -a"  and click at xkb-plugin but nothing happens on the panel
  
xfce4-panel writes at command line:
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(xfce4-xkb-plugin:1637): GLib-CRITICAL **: g_hash_table_destroy: assertion `hash_table != NULL' failed

(xfce4-xkb-plugin:1637): GLib-CRITICAL **: g_hash_table_destroy: assertion `hash_table != NULL' failed

(xfce4-xkb-plugin:1637): GLib-CRITICAL **: g_hash_table_destroy: assertion `hash_table != NULL' failed
The program 'xfce4-xkb-plugin' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadClass, invalid event class'.
  (Details: serial 208 error_code 133 request_code 131 minor_code 6)
  (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.)
libxfce4panel-Message: Keyboard Layouts: screen changed: 0

libxfce4panel-Message: No valid plug window.

(xfce4-panel:1634): Gtk-CRITICAL **: gtk_socket_get_id: assertion `GTK_WIDGET_ANCHORED (socket)' failed

(xfce4-panel:1634): libxfce4panel-CRITICAL **: An item was unexpectedly removed: "Keyboard Layouts".

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I also tried to install newest version of xkb-plugin but have the same result 

XFce-4.6, HAL is disabled