Probleem met insert formulier

Ik heb de volgende functie in mijn Model:

	public function admin_toevoegen($name, $email, $password)
	{
		$sql = "INSERT INTO `site_admins` (`name`, `email`, `password`) 
			         VALUES (?, ?, ?)";
			   
		$stmt = $this->pdo->prepare($sql);
        $stmt->execute(array($name, $email, $password));
	}

en deze in de Controller:

	public function admin_toevoegenAction()
	{
		if ($_SERVER['REQUEST_METHOD'] != 'POST') 
		{
			$this->page->redirect('/beheerder_toevoegen');
		} else {
		
 			$name            = filter_input(INPUT_POST, 'a_name', FILTER_SANITIZE_STRING);
			$email           = filter_input(INPUT_POST, 'a_email', FILTER_SANITIZE_STRING);
			$password        = filter_input(INPUT_POST, 'a_password', FILTER_SANITIZE_STRING);
			$hashed          = password_hash($password, PASSWORD_DEFAULT);
			
			$this->page->admin_toevoegen($name, $email, $hashed);
			
		}
			
	}

En dit i het formulier in kwestie:

            	<form role="form" method="post" class="adminForm" id="adminForm" action="/admin_toevoegen">
                	<div class="form-group mt-1">
                    	<input type="text" class="form-control" name="a_name" id="a_name" placeholder="name *" tabindex="1">
                    </div>
                	<div class="form-group mt-1">
                    	<input type="text" class="form-control" name="a_email" id="a_email" placeholder="email *" tabindex="1">
                    </div>
                	<div class="form-group mt-1">
                    	<input type="text" class="form-control" name="a_password" id="a_password" placeholder="password *" tabindex="1">
                    </div>
					<div class="form-group mt-1">
						<input type="submit" class="btn btn-medium btn-cta" name="submit" value="Subscribe now!" tabindex="1">
					</div>
                </form>

Maar als ik het formulier submit gebeurt er helemaal niets en de console geeft de volgende melding:

Failed to load resource: the server responded with a status of 503 (Service Unavailable)

Alle andere pagina's werken prima. Wat kan de oorzaak zijn?

Edit: Dit is de pagina in kwestie http://www.flying-paradise.com/beheerder_toevoegen