expatreader exception during xhtml

I'm getting the following error message...

Error while evaluating the expression "xhtml(HTML)" defined in the "from" part of a statement.
File "<string>", line 1, in <module>
File "/var/snapshots/appy-current/appy/pod/renderer.py", line 240, in renderXhtml
stylesMapping, ns).run()
File "/var/snapshots/appy-current/appy/pod/xhtml2odt.py", line 501, in run
self.xhtmlParser.parse(self.xhtmlString)
File "/var/snapshots/appy-current/appy/shared/xml_parser.py", line 193, in parse
self.parser.parse(inputSource)
File "/usr/lib/python2.6/xml/sax/expatreader.py", line 107, in parse
xmlreader.IncrementalParser.parse(self, source)
File "/usr/lib/python2.6/xml/sax/xmlreader.py", line 123, in parse
self.feed(buffer)
File "/usr/lib/python2.6/xml/sax/expatreader.py", line 211, in feed
self._err_handler.fatalError(exc)
File "/usr/lib/python2.6/xml/sax/handler.py", line 38, in fatalError
raise exception
<class 'xml.sax._exceptions.SAXParseException'>: <unknown>:1:1: not well-formed (invalid token)

It comes in the document `test_result_4.odt` produced by my test.py script at
http://code.google.com/p/lino/source/browse/tests/appy/1/test.py

Funnily the error occurs only on the customer's Debian box (Python 2.6) and not on my Windows XP notebook (Python 2.7 or 2.6). Any ideas on what might be the explanation and how to get a more useful error message?