P.S. Accessing the servlets through the HTTP listener did not cause HTTP/500 errors - so the glitch is definitely in the AJP13 listener.
get the latest Jetty release from www.sourceforge.net.
substitute the jars in your deploy/jbossweb.sar with the corresponding ones in the lib dir of this release (you may have to try both org.mortbay.jetty.jars - one is for JVM-1.2 I think).
If the problem persists, please come straight back to me.