Php HTTPS protocol page does not redirect

Good afternoon! I am having a problem, when the login expires the page is blank and the command

header("location: login.html"); 
exit(); 

does not work. This only happens on the site with HTTPS. Can someone give a light?

$valor = isset($_SESSION['MM_Admin']) ? 'S' : 'N'; 
if($valor == 'N') {
  header("location: login.html"); exit();
}