ClassCastException with Servlets

When I try to run any of my own (or foreign) servlets under Jigsaw I get a
ClassCastException:
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Servlet has thrown exception:java.lang.ClassCastException:
org.w3c.jigsaw.servlet.JigsawHttpServletRequest
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I got no idea what is going on here. Any hints???
I'm using Jdk 1.1.7, Jsdk 2.0 and Jigsaw release 2.0.1


Thank you in advance,
Werner