I have had similar issues with other internal libraries and its worth noting that based on whats overriding your classes, the classloader policy may need to be set at a different scope.
Did you set PARENT_LAST at the application scope or vm scope? Try different combinations, and its also useful to enable verbose class loading so you can see whats being loaded from where.
I'm not sure if the IBM 1.5 jdk supports -verbose:class option but that would do the same thing as checking the "verbose class loading" box in the java virtual machine properties.
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