Ajax错误的请求（也是另一个脚本php ajax脚本）

I'm having a really weird problem. My jQuery Ajax request is ALSO going to another php ajax script (must go to ajax/sign-in.php and give me an error in ajax/users.php) (without any redirect code) and it's giving me strange errors.

Basically, i have the jQuery Ajax Request:

login = function(){
    var data = {
        login    : $('input[name="login"]').val(),
        password   : $('input[name="password"]').val()
    }

    $.ajax({
        type: "POST",
        url: '../ajax/signin.php',
        dataType: 'json',
        data: data,
        success: function(result){
            addLoading('.signIn button');
            if (result.success){
                //window.location="//aquickgig.com";
                console.log('huehue');
            }else{
                setError(".signIn button", result.error);
            }
            removeLoading('.signIn');
        },
        error: function(xhr, ajaxOptions, thrownError){
            alert("Error Status: " + xhr.status + " Thrown Errors: "+thrownError);
        }
    });
}

And my Ajax php script:

<?php
header('Cache-Control: no-cache, must-revalidate');
header('Expires: Mon, 01 Jan 1996 00:00:00 GMT');
header('Content-Type: application/json');
include_once($_SERVER['DOCUMENT_ROOT'] . '/lib/users.php');
$users = new Users();

isset($_POST['login']) ? $login = $_POST['login'] : $login = null;
isset($_POST['password']) ? $password = $_POST['password'] : $password = null;
$resp = array();

if (strlen($login) == 0 || strlen($password) == 0){
    $resp['error'] = "Please, fill all the fields";
}
elseif (strlen($login) > 50 || strlen($password) > 50){
    $resp['error'] = "Invalid number of characters. Usernames and password cannot be above 50 characters";
}
else{
    $resp['success']  = "something";
}
echo json_encode($resp);

?>

When i do the request, the console doesn't give me scripts errors and it does the request. But on the catch, give me the answer: Error Status 200 Thrown Errors: SintaxeError: Unexpected token <.

Also, the response from request is Fatal error: Class 'Users' not found in C:\xampp\htdocs\ajax\users.php on line 8. I NEVER called the users.php script.

Trying to figure out the problem, i noticed that when i remove or comment the line include_once($_SERVER['DOCUMENT_ROOT'] . '/lib/users.php'); from sign-in.php, it works perfectly.

I really confused, don't know what's happening. Thank you!