VB Login api problem

[sulanum]

Hello,

I want to use the login api, but it gives me this fatal error :

Fatal error: Class 'vB5_Frontend_Application' not found in /home/blabla/domains/mydomain.com/public_html/forums/serverintegration.php on line 12

<?php
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

// Path to your vBulletin installation
$vbpath = '';

// Start login script
define('CSRF_PROTECTION', false);
require_once(__DIR__ . '/includes/vb5/autoloader.php');
vB5_Autoloader::register($vbpath);
vB5_Frontend_Application::init('config.php');

if ($_SERVER['REQUEST_METHOD'] == 'POST')
{
    ///////////////////////////////////////////////////////////////////////////////////////
    // process the login form

    $api = Api_InterfaceAbstract::instance();
    $loginInfo = $api->callApi('user', 'login', array($_POST['username'], $_POST['password']));

    if (empty($loginInfo['errors']))
    {
        // set cookies
        vB5_Auth::setLoginCookies($loginInfo, '', !empty($_POST['remember']));

        // redirect somewhere Also see: vB5_Auth::doLoginRedirect();
        header('Location: vb5_external_login.php');
        exit;
    }
    else
    {
        // there was a problem logging in.
        // redirect or display errors here
    }
}
else
{
    ///////////////////////////////////////////////////////////////////////////////////////
    // display a login form

    $userid = vB5_Cookie::get('userid', vB5_Cookie::TYPE_UINT);
    $hash = vB5_Cookie::get('password', vB5_Cookie::TYPE_STRING);

    if (empty($userid) OR empty($hash))
    {
        ?>
        <form action="vb5_external_login.php" method="post">

            <input type="text" name="username" value="" placeholder="User Name" />
            <input type="password" name="password" value="" placeholder="Password" />
            <label><input type="checkbox" name="remember" /> Stay logged in?</label>
            <input type="submit" value="Log In" />

        </form>
        <?php
    }
    else
    {
        echo 'Already logged in';
    }
}

?>

Any ideas what the problem is, and how to fix it?