Jetty http session is always null (Embedded Container, ServletHolder)

I am trying to implement a simple servlet which uses a HTTP session in an embedded jetty (7.3.0 v20110203) container. To start jetty I use the following code:

Server server = new Server(12043);
ServletContextHandler handler = new
            ServletContextHandler(ServletContextHandler.SESSIONS);
handler.setContextPath("/");
server.setHandler(handler);
ServletHolder holder = new ServletHolder(new BaseServlet());
handler.addServlet(holder, "/*");
server.start();
server.join();

The servlet acquires a session with

HttpSession session = request.getSession(true);

and stores some data in it. Upon the next request it gets the session with the following code:

HttpSession session = request.getSession(false);

and there the session is always null.

I did not find any information on the internet about this particular problem. I have also experimented with setting a SessionManager or SessionIdManager, but that did not seem to change anything. I suspect I am missing something about SessionManager or SessionIdManager or SessionHandler here, but this is just a wild guess.