Cloud.User.logout returns 406 Not Acceptable

I use ACS (Appcelerator Cloud Services) in my mobile application. It works fine to log in via Facebook, Twitter and as well with ACS e-mail and password. But when I call logout I get the following error:

HTTP/1.1 406 Not Acceptable

Does anybody recognize this?

Code used:

Cloud.Users.logout(function(response){
    if (response.success){
        alert("you are logged out");
    } else {
        alert("failed logout: " + response.message);
    }
});