java.lang.ClassCastException: com.google.apphosting.utils.jetty.AppEngineWebAppContext $AppEngineServletContext cannot be cast to org.mortbay.jetty.handler.ContextHandler$SContext

Quoting the solution from here,

Did you accidentally include appengine-local-runtime.jar in your
WEB-INF/lib?

So removing it solves the problem.