Page redirection with inconsistent JS and PHP

Question

I am trying to redirect to a page using window.location.replace(). However, sometimes the page does not redirect. At first I thought the error was loading a google source, but even removing, the problem persisted.

JS/jQuery code

$('#button_login').on('click', function() {
    var login = $('#inputLogin').val();
    var pass = $('#inputPassword').val();
    var remember = $('#remember').is(':checked') ? $('#remember').val() : 'off';


    if (login != '' && pass != '') {
        $.ajax({
            url: 'functions/signin.php',
            method: 'POST',
            cache: false,
            data: {
                login: login,
                pass: pass,
                remember: remember
            },
            success: function(data, status, request) {
                if (data == "OK") {
                    window.location.replace("index.php");

                } else {

                }
            },
            error: function(request, status, error) {

            }
        });
    } else {
        $('#inputLogin')[0].oninvalid = function(event) {
            event.preventDefault();
        }
        $('#inputPassword')[0].oninvalid = function(event) {
            event.preventDefault();
        }
        $('#alert-login').html('Preencha os campos');
        $('#alert-login').fadeTo(2000, 500).slideUp(1000, 
            function() {
                $('#alert-login').slideUp(500);
            });
    }
});

function getCookie(cname) {
    var name = cname + "=";
    var decodedCookie = decodeURIComponent(document.cookie);
    var ca = decodedCookie.split(';');
    for(var i = 0; i <ca.length; i++) {
        var c = ca[i];
        while (c.charAt(0) == ' ') {
            c = c.substring(1);
        }
        if (c.indexOf(name) == 0) {
            return c.substring(name.length, c.length);
        }
    }
    return "";
}

$(document).ready(function() {
    var login = getCookie('login');
    var pass = getCookie('pass');
    var remember = getCookie('remember');
    if (remember != '') {
        $('#inputLogin').val(login);
        $('#inputPassword').val(pass);
        $('#remember').click();
    }
});

PHP code

<?php
    require_once('../config.php');
    require_once('../'.DBAPI);
    require_once('../'.SESSIONAPI);

    if (isset($_POST['login'])) {
        $session = Session::getInstance();
        $db = open_database();

        /* Recuperação dos dados do login */
        $login = $_POST['login'];
        $pass = $_POST['pass'];
        $remember = $_POST['remember'];

        $sql = "SELECT * FROM Users WHERE login = '$login' AND pass = '$pass'";

        try {
            $result = $db->query($sql);
            $row = $result->fetch_assoc();
            if ($result->num_rows == 1) {
                $session->advocad = 'yes';
                $session->username = $row['username'];
                /* Criar os cookies */
                if ($remember == 'on') {
                    setcookie("login", $login, time() + (86400 * 30 * 12), "/"); /* nome_cookie, valor_cookie, duracao_cookie, acesso */
                    setcookie("pass", $pass, time() + (86400 * 30 * 12), "/");
                    setcookie("remember", $remember, time() + (86400 * 30 * 12), "/");

                } else {
                    setcookie("login", "", time() -3600, "/");
                    setcookie("pass", "", time() -3600, "/");
                    setcookie("remember", "", time() -3600, "/");
                    //var_dump($_COOKIE);
                }
                $auth = "OK";

            } else {
                $session->destroy();
                $auth = "NO";
            }

        } catch (Exception $e) {
            unset($_SESSION['advocad']);
            $auth = $e->GetMessage();
        }

        close_database($db);
        echo $auth;
    }
?>