A "Valid" XML document is a "Well Formed" XML document, which also conforms to the rules of a Document Type Definition (DTD). The purpose of a DTD is to define the structure of an XML document. It defines the structure with a list of legal elements.
Details can be read here:
http://www.w3schools.com/xml/xml_dtd.asp[
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http://en.wikipedia.org/wiki/Document_Type_Definition[
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It looks like you have your own DTD defined for the XML's that you are using. While loading, those DTD's are expected to be present to validate the XML.
Either provide it or remove the DTD reference (full DOCTYPE tag) from your XML.