AJAX being difficult and rude.

[kh99]

You might try disabling all plugins, there maybe be some in global.php and other files included by that.

Then I don't know but if I were trying to figure it out I'd probably put some code in ajax.php (or create a plugin on one of those hooks) to see if it's actually being output by the call to print_xml() (like maybe write the output to a file) although looking over the code briefly, I don't see how that would be happening (you probablly came to the same conclusion).