I finally got around to test it with the latest configuration by removing the old jetty-context.xml and adding the new configuration file in the Jetty webapps directory. Unfortunately the error remains the same.

testwebapp-1.0.0.xml

<?xml version="1.0"?>

<!DOCTYPE Configure PUBLIC "-//Mort Bay Consulting//DTD Configure//EN" "http://www.eclipse.org/jetty/configure_9_0.dtd">

<Configure class="org.eclipse.jetty.webapp.WebAppContext">

  <Set name="contextPath">/webapp</Set>

  <Set name="war">./webapps/testwebapp-1.0.0.war</Set>

  <Call name="prependServerClass">

    <Arg>-org.eclipse.jetty.server.handler.ContextHandler,-org.eclipse.jetty.servlet.FilterHolder,-org.eclipse.jetty.servlet.ServletContextHandler,-org.eclipse.jetty.servlet.ServletHolder</Arg>

  </Call>

</Configure>

Still the same error:

2015-09-14 10:15:06.396:WARN:oejd.DeploymentManager:main: Unable to reach node goal: started
java.lang.NoClassDefFoundError: org/eclipse/jetty/servlet/ServletContextHandler$Decorator
[...]

Tried it this time with the latest version 2.3.0.CR2.