In spite of having a valid xml the loader.xmlDoc.responseXML returns a null value … Firefox firebug returns a valid XML object in the response header,however the response XML is null when calling through an dhtmlxAjax.post() ??
Can you please provide snippet of code which you are using for data loading?
The post method is async, which may be a reason of problem, also beware that in case of XML, response must not contain any whitespaces before <?xml declation, or browser will not be able to parse it as XML ( in such case responseText will available but not the responseXML )