This error probably means that your XML file is not valid according to the Mediawiki DTD (http://en.wikipedia.org/wiki/Document_Type_Definition) file.
Might be a problem in your XML file or in the version of it.
Which version of Mediawiki you are running? Can you tell me the version of the XML file (on the top of the XML file look for something like "http://www.mediawiki.org/xml/export-0.3/")?
The importer was tested with Mediawiki 1.14 but it is very likely that it works with other versions.
Maybe they have updated their XML definition, if no structural changes have been made should be very easy to support this new version.