method ZKLessServlet.doGet():

String reqFilePath = request.getPathInfo(); - this method returns reqFilePath including leading slash {e.g. "/window.less")
String reqRealPath = sc.getRealPath(lessResource + "/" + reqFilePath); - concatenates requested path with double slash (e.g. "/less//window.less")

On Jetty 9 the reqRealPath is then null. If double slash is removed, everything works correctly.