Hi Alberto,
ok, in that example there's a menu item with more than 1 parameter. Sounds wierd I know, but that was not xHtml complaint, and caused the xml response to fail. you can see this in Firebug, in the console, in the xml tab of the response.
I have tweaked 6.39 so that menu items with more than 1 parameter are xHtml complaint, which will solve this problem.
Cheers
Bruce