Nodejs express session memorystore cannot find session with given Id

I've followed tutorials on how to create a node express app with cookie parsing and a MemoryStore as Session store. But after installing the latest versions of some modules my app isn't working anymore. Yesterday I installed the latest versions of "express", "connect" and "cookie" and now I cannot get the sessions out of the MemoryStore anymore.

Below is the simple app that I've set up to reproduce the problem:

server.js -----------

var express = require('express');
var MemoryStore = express.session.MemoryStore;
var sessionStore = new MemoryStore();
var connect = require('connect');
var Session = connect.middleware.session.Session;
var cookie = require('cookie');

module.exports.startServer = function() {

var app = express();

// Configuration
app.configure(function() {
    app.use(express.bodyParser());
    app.use(express.methodOverride());
    app.use(express.cookieParser());
    app.use(express.session({
        store : sessionStore,
        secret : 'secret',
        key : 'express.sid'
    }));
    app.use(express.static(__dirname + '/public'));
    app.use(app.router);
});

app.configure('development', function() {
    app.use(express.errorHandler({
        dumpExceptions : true,
        showStack : true
    }));
});

app.configure('production', function() {
    app.use(express.errorHandler());
});



// Init routes
app.post('/login', function(req, res){
    var credentials = req.body;

    if (!(credentials.username && credentials.password)){
        res.redirect('/login.html');
        return;
    } 

    if (credentials.username === 'user1' && credentials.password === 'pass1'){
        req.session.user = credentials.username;
        req.session.clientId = credentials.clientId;
        res.redirect('/post-message.html');
    }else{
        req.session.destroy();
        res.redirect('/login.html');
    }
});

app.post('/postMsg', authenticate, function(req, res){
    res.send('posted');
});

app.listen(4000);

function authenticate(req, res, next) {
    // check if there's a cookie header
    if (req.headers.cookie) {
        // if there is, parse the cookie
        req.cookie = cookie.parse(req.headers.cookie);

        req.sessionID= req.cookie['express.sid'];
        // note that you will need to use the same key to grad the
        // session id, as you specified in the Express setup.

        sessionStore.get(req.sessionID, function(err, session) {

            if (session && session.user) {
                // save the session data and accept the connection
                req.session = new Session(req, session);
                next();
            }
            else {
                //Turn down the connection
                res.redirect('/login.html');
            }
        });
    } else {
        // if there isn't, turn down the connection with a message
        // and leave the function.
        res.redirect('/login.html');
    }
}
};

I can see in the debugger that everything seems to work fine until I'm trying to post a message by calling the "/postMsg" route. Then it enters the "authenticate"-function and tries to get the session with the "req.sessionID" from the sessionStore. That does not suceed anymore and the sessionStore.get returns undefined for the session. But if I look in the sessionStore using the debugger I can see that there is a session in the store and it also seems to match the sessionID.

Does anyone know what's wrong with my script?

Thanks for help!