$.ajax submiting a form, sometimes get success and sometimes error

I have a simple log-in form that passed to the php through $.ajax function. The problem is that on localhost the $.ajax function result is sometimes success and sometimes error. most of the time when i'm getting success is when i'm using chrome debugger. When I checked the file on a server i only got error result by the $.ajax.

Thanks in advance..

form code:

<form method="post" action="">
        <h3>Login</h3>
        <label>User Name:<input type="text" name="uname" id="uname"></label>
        <br>
        <label>Password:<input type="password" name="pass" id="pass"></label>
        <br>
        <button type="submit" id="submit">login</button>
    </form>

$.ajax code

  $("#submit").click(function(){
                $.ajax({
                    cache: false,
                    url: 'php/login.php',
                    type: 'POST',
                    dataType: 'json',
                    data: {
                        uname: $('#uname').val(),
                        pass: $('#pass').val()
                    },

                    success: function (data) {
                        Cookies.set('uid', data[0].uid);
                        alert("test");
                    },
                    error: function (xhr, status) {
                        alert("Sorry, there was a problem!");
                    },
                });
            })

php code

header("Access-Control-Allow-Origin: *");
header("Content-Type: application/json; charset=UTF-8");

require_once 'config.php';
$conn = mysqli_connect($hn, $un, $pw, $db);
if ($conn->connect_error) die($conn->connect_error);

$uname=mysql_real_escape_string($_POST['uname']);
$pass=md5(mysql_real_escape_string($_POST['pass']));

$result = $conn->query("SELECT * FROM users WHERE uname LIKE '$uname' AND upass LIKE '$pass'");     

    $outp = "[";
    while($rs = $result->fetch_array(MYSQLI_ASSOC)) {
            $outp .= '{"uid":"'.$rs["uid"].'"}';
        }
        $outp .="]"; 

$conn->close();
echo($outp);