CakePHP 3 Authentication on Production - cakephp

Question

I have followed pretty much the tutorial from the CakePHP website to add authentication to my website (except that I do not need any roles on my website, but just an admin login). On my machine the whole authentication works and also which pages I can access only as admin. I have copied the folder and the database to my web hoster, but there I have problems logging in.
This is what my code looks like:
in config/routes.php I have:
Router::prefix('admin', function($routes) {
$routes->connect('/', ['controller'=>'Users','action'=>'login']);
$routes->fallbacks('InflectedRoute');
});
in src/Controller/Admin/UsersController.php
namespace App\Controller\Admin;
use App\Controller\AppController;
use Cake\Event\Event;
class UsersController extends AppController {
public function beforeFilter(Event $event) {
parent::beforeFilter($event);
$this->Auth->allow(['logout']);
}
public function login(){
if ($this->request->is('post')) {
$user = $this->Auth->identify();
if ($user) {
$this->Auth->setUser($user);
return $this->redirect($this->Auth->redirectUrl());
}
$this->Flash->error(__('Please check your username and password.'));
}
}
}
The entity User contains a method to do the hashing:
protected function _setPassword($password)
{
return (new DefaultPasswordHasher)->hash($password);
}
Finally, in Controller/AppController.php I have implemented the following, that should allow every logged-in user to access all pages.
class AppController extends Controller
{
public function initialize()
{
parent::initialize();
$this->loadComponent('Flash');
$this->loadComponent('Auth', [
'loginRedirect' => [
'controller' => 'requests',
'action' => 'index'
],
'logoutRedirect' => [
'prefix' => false,
'controller' => '/'
]
]);
}
public function beforeFilter(Event $event)
{
parent::beforeFilter($event);
if($this->request->session()->check('Auth.User.id')) {
$this->layout = 'admin';
}
}
public function isAuthorized($user)
{
return true;
}
}
As I said, this works on my local machine. However, on the server, I always get the following error:
You are not authorized to access that location.
I tried various PHP versions, to make sure the error is not there. Using printouts, I could see that I pass the if($user) check in the login() method. However, now I really don't know how to proceed and I'm pretty clueless about possibly solutions. I have installed CakePHP in a subdomain on the webhoster - could that be a problem?