SaxParser on Tomcat 6 HPUX Itanium 11.31

[Joc]

Have to load old xerces and xalan libs under Tomcat 6 in HPUX 11.31 . The install went fine, and the servlet is installed as a standard tomcat 6 servlet. However, well into the application it fails with
org.jdom.JDOMException: SAX2 driver class org.apache.xerces.parsers.SAXParser do
es not implement XMLReader: SAX2 driver class org.apache.xerces.parsers.SAXParser

But XMLReader is definately implemented in org.apache.xerces.parsers.SaxParser.class, which resides in xerces.jar, in the webapps own WEB-INF/lib directory. According to Tomcat6 classloader Howto, this is where it should be. Also tried to endorse the lib according to java 1.5 endorsement, to no avail. Have tried with the latest Xerces and Xalan libs, but they produced other errors.
The application is ported from HPUX PA-RISC, where it does work with the abovementioned 'old' jar files. I seem to be out of options here . Any suggestion is appreciated