Expressjs doesn't destroy session

I have an Backbone View which sends an Ajax call to the server to remove a session.

On the server following event is triggered:

app.delete('/session', function(req, res) {
    if (req.session) {
        req.session.destroy(function() {
            res.clearCookie('connect.sid', { path: '/' });
            res.send('removed session', 200);
        });
    } else {
        res.send('no session assigned', 500);
    }
});

The weird about this is that I can press the logout button multiple times without getting a HTTP 500 error code. Also chromium shows me that a cookie still exists.

What is going wrong?

Regards

EDIT:

I found out that this isn't directly a session issue but a cookie one. I added res.clearCookie to the route. Unfortunatly the behaviour (cookie, session keep alive) didn't change

EDIT2: I now gave res.clearCookie some parameters => res.clearCookie('connect.sid', { path: '/' }); Now at least the cookie is gone in the browser. But the session seems to be still available. Or at least I can call the logout route how often I want even req.session should be false

EDIT3: I now removed all sessions out of redis and restarted everything (redis, node, browser). Than I have logged in again and logged out. This works so far but when I relaod the page with F5 I get a new session. WHY?