To be safe, turn off all routing for cakePHP when it accesses the xd_receiver.htm file.
/**
* turn off cakephp routing for facebook connect xd_receiver
*/
Router::connect('/xd_receiver.htm', null);
Some interesting stuff off the beaten track. It could be physical, mental or mathematical.
/**
* turn off cakephp routing for facebook connect xd_receiver
*/
Router::connect('/xd_receiver.htm', null);