The XMLHttpRequest() succeeds cause there's nothing wrong with making the local call. it's just different and the problem is in the buggy mootools isSuccess function.
You gotta override it the Request options. Here's how jquery does it
// Determines if an XMLHttpRequest was successful or not
httpSuccess: function( xhr ) {
try {
// IE error sometimes returns 1223 when it should be 204 so treat it as success, see #1450
return !xhr.status && location.protocol === "file:" ||
// Opera returns 0 when status is 304
( xhr.status >= 200 && xhr.status < 300 ) ||
xhr.status === 304 || xhr.status === 1223 || xhr.status === 0;
} catch(e) {}
return false;
},