AjaxSetup never execute the success function

I have a simple registration page that validates if a user is already taken.

I use ajaxSetup for all my ajax calls and for some reason the "success" is never called. When I look at my console (firebug) I can see a successfully request (code 200 OK and I get true or false as the result).

Here's my code:

$('#checkValidUsername').click(function() {
    // some basic validation like not empty etc...
    $.ajax({
        type: "POST",
        url: '/checkuser.php',
        cache: false,
        data: $("#form").serialize(),
        dataType: 'json',
        success: function(result) {
            // do some actions
        },
    });
}

$.ajaxSetup({
    beforeSend: function() {
        // show loading dialog // works
    },
    complete: function(xhr, stat) {
        // hide dialog // works
    }
    success: function(result,status,xhr) {
        // not showing the alert
        alert('success');
    }
});

What is wrong with my code? Thank you