I’ve a controller who manage a password reset form submit, everything works fine except setting the statusmessage or any other message. I’ve tried to alter the "message" in the .module file too with no luck. It maybe has something to do with migrating to drupal 9, on drupal 8 it worked. here is the code:

    <?php          namespace Drupalcustom_form_saveController;          use SymfonyComponentHttpFoundationResponse;     use DrupalCoreSessionAccountProxyInterface;     use DrupaluserUserInterface;     use DrupalCoreImageImageFactory;     use DrupaluserEntityUser;     use DrupalnodeEntityNode;     use DrupalfileEntityFile;     use DrupalCoreEntityEntityManagerInterface;     use DrupalCoreEntityEntityInterface;     use DrupalcommentEntityComment;     use DrupalCoreControllerControllerBase;     use DrupalblockEntityBlock;     use DrupalCoreBlockBlockBase;       use DrupalCoreSessionAccountInterface;     use SymfonyComponentHttpFoundationRequest;     use DrupalnodeNodeInterface;     use DrupalimageEntityImageStyle;     use SymfonyComponentHttpFoundationRedirectResponse;     use DrupaluserPrivateTempStoreFactory;     use SymfonyComponentDependencyInjectionContainerInterface;     use Drupalcustom_form_saveControllerEmailsController;     use Drupalcontent_managerControllerStaticController;     use DrupalCoreMessengerMessengerInterface;          class ResetPasswordSendController extends ControllerBase {              public function resetpasswordsend(){                  $static_functions = new StaticController();             $routes = $static_functions->urlmanagerwebsite();                          $session = Drupal::service('session');             if (!$session->isStarted()) {                 $session->migrate();             }                  $host = Drupal::request()->getSchemeAndHttpHost();             $name = $_POST['email'];             $langcode =  Drupal::languageManager()->getCurrentLanguage()->getId();             $users = Drupal::entityTypeManager()->getStorage('user')->loadByProperties(array('mail' => $name));             if (!empty($users)) {                 $account = reset($users);                 $mail = _user_mail_notify('password_reset', $account, $langcode);                 Drupal::messenger()->addMessage('Wir haben eine Anleitung zum Zurücksetzen des Passworts an deine registrierte E-Mail-Adresse gesendet.'); //NOT WORKING                 $response = new RedirectResponse($host.$routes['resetpassword']);                 $response->send();                 exit;             }                  $addmessage = $name.' wird nicht als E-Mail-Adresse erkannt.';             $this->messenger()->addMessage($addmessage); //NOT WORKING             $session->set('messagescustom', $addmessage); //NOT WORKING             Drupal::messenger()->addMessage('Die eingegebenen E-Mail Adresse stimmen nicht überein.'); //NOT WORKING             $response = new RedirectResponse($host.$routes['resetpassword']);             $response->send();             exit;         }     } 

The theme should not be the problem, because on every other page messages are working.