Laman tidak teralihkan dengan benar

Halo gan, setiap kodenya seperti ini, login berjalan mulus

<?php
/**
 * this file is used for register a new user admin.
 */
require_once "../Core/init.php";

if (Session::exists('examygoUser')) {
    Redirect::to('/examygo/dashboard/');
}

if (Input::get('submit')) {

    if (Helper::checkToken(Input::get('csrf'))) {
        //menjaga isi form tidak hilang saat validasi gagal
        $_SESSION = $_POST;
        //panggil kelas validasi
        $validasi = new Validation;

        //rule validasi
        $validasi = $validasi->check([
            'username'  => ['required'  =>   true],
            'password'  => ['required'  =>   true]
        ]);
        //mengecek apakah lolos dari error!
        if ($validasi->passed()) {
            //memasukkan data ke database
            if ($user->loginUser(Input::get('username'), Input::get('password'))) {
                // Session::delete('nama');
                //make a session for login authentication
                Session::set('examygoUser', Input::get('username'));
                // if (Session::exists('lastPage')) {
                //     $lastPage = Session::get('lastPage');
                //     Session::delete('lastPage');
                //     Redirect::to('http://localhost' . $lastPage);
                // } else {
                Redirect::to('/examygo/dashboard/');
                //}
            } else {
                $errors['username'] = 'Login gagal!';
            }
        } else {
            $errors = $validasi->errors();
        }
    } else {
        die('oops!, token is not valid');
    }
}

if (Session::exists('examygoFlash')) {
    $errors['username'] = Session::flash('examygoFlash');
}

require_once "../Views/Templates/header.php";
?>

<div class="row justify-content-center m-2">
    <div class="col my-boxes border p-3 rounded bg-white shadow-lg mt-3">
        <h2 class="center">Login</h2>
        <hr />
        <form action="/examygo/user/login.php" method="POST">
            <div class="form-group">
                <label for="username">Username<span class="text-danger">*</span></label>
                <?php if (!empty($errors['username'])) : ?><div class="alert alert-danger"><?= $errors['username']; ?></div><?php endif; ?>
                <input type="text" class="form-control" name="username" id="username" value="<?= @$_SESSION['username'] ?>" required />
            </div>
            <div class="form-group">
                <label for="password">Password<span class="text-danger">*</span></label>
                <?php if (!empty($errors['password'])) : ?><div class="alert alert-danger"><?= $errors['password']; ?></div><?php endif; ?>
                <input type="password" class="form-control" name="password" id="password" required />
            </div>
            <input type="hidden" name="csrf" value="<?= Helper::generateToken() ?>" />
            <input class="btn btn-primary btn-block" type="submit" name="submit" value="Submit" />
        </form>
    </div>
</div>

<?php require_once "../Views/Templates/footer.php"; ?>

tapi, kalo saya ubah $_SESSION = $_POST ke $_SESSION['param'] = $_POST['param'] muncul error Laman tidak teralihkan dengan benar. berikut kode yang error

<?php
/**
 * this file is used for register a new user admin.
 */
require_once "../Core/init.php";

if (Session::exists('examygoUser')) {
    Redirect::to('/examygo/dashboard/');
}

if (Input::get('submit')) {

    if (Helper::checkToken(Input::get('csrf'))) {
        //menjaga isi form tidak hilang saat validasi gagal
        $_SESSION['vvv'] = $_POST['username'];
        //panggil kelas validasi
        $validasi = new Validation;

        //rule validasi
        $validasi = $validasi->check([
            'username'  => ['required'  =>   true],
            'password'  => ['required'  =>   true]
        ]);
        //mengecek apakah lolos dari error!
        if ($validasi->passed()) {
            //memasukkan data ke database
            if ($user->loginUser(Input::get('username'), Input::get('password'))) {
                // Session::delete('nama');
                //make a session for login authentication
                Session::set('examygoUser', Input::get('username'));
                // if (Session::exists('lastPage')) {
                //     $lastPage = Session::get('lastPage');
                //     Session::delete('lastPage');
                //     Redirect::to('http://localhost' . $lastPage);
                // } else {
                Redirect::to('/examygo/dashboard/');
                //}
            } else {
                $errors['username'] = 'Login gagal!';
            }
        } else {
            $errors = $validasi->errors();
        }
    } else {
        die('oops!, token is not valid');
    }
}

if (Session::exists('examygoFlash')) {
    $errors['username'] = Session::flash('examygoFlash');
}

require_once "../Views/Templates/header.php";
?>

<div class="row justify-content-center m-2">
    <div class="col my-boxes border p-3 rounded bg-white shadow-lg mt-3">
        <h2 class="center">Login</h2>
        <hr />
        <form action="/examygo/user/login.php" method="POST">
            <div class="form-group">
                <label for="username">Username<span class="text-danger">*</span></label>
                <?php if (!empty($errors['username'])) : ?><div class="alert alert-danger"><?= $errors['username']; ?></div><?php endif; ?>
                <input type="text" class="form-control" name="username" id="username" value="<?= @$_SESSION['vvv'] ?>" required />
            </div>
            <div class="form-group">
                <label for="password">Password<span class="text-danger">*</span></label>
                <?php if (!empty($errors['password'])) : ?><div class="alert alert-danger"><?= $errors['password']; ?></div><?php endif; ?>
                <input type="password" class="form-control" name="password" id="password" required />
            </div>
            <input type="hidden" name="csrf" value="<?= Helper::generateToken() ?>" />
            <input class="btn btn-primary btn-block" type="submit" name="submit" value="Submit" />
        </form>
    </div>
</div>

<?php require_once "../Views/Templates/footer.php"; ?>

Tampilan error nya