Two Oracle Homes installed on same AIX - opatch only works in one

 Hi, we are running Oracle Databae 10.2.0.4 on AIX6. We have installed two Oracle Homes each under their own OS user.
In one of the Oracle Homes opatch works correctly and has been used for
installing the latest CPU.
In the other home opatch throws following error whenever opatch
is requested to execute a command (like lsinventory):
javax.xml.parsers.FactoryConfigurationError: Provider oracle.xml.jaxp.JXDocumentBuilderFactory not found
        at javax.xml.parsers.DocumentBuilderFactory.newInstance(Unknown Source)
        at oracle.opatch.CmdLineOptionsManager.loadCmdLineOptionsXML(CmdLineOptionsManager.java:82)
        at oracle.opatch.CmdLineOptionsManager.<init>(CmdLineOptionsManager.java:60)
        at oracle.opatch.OPatchCmdLineParser.<init>(OPatchCmdLineParser.java:72)
        at oracle.opatch.CmdLineParser.parse(CmdLineParser.java:352)