app/bundles/UserBundle/Security/Authentication/AuthenticationHandler.php

<?php

namespace Mautic\UserBundle\Security\Authentication;

use Symfony\Component\HttpFoundation\RedirectResponse;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\RouterInterface;
use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
use Symfony\Component\Security\Core\Exception\AuthenticationException;
use Symfony\Component\Security\Core\Security;
use Symfony\Component\Security\Http\Authentication\AuthenticationFailureHandlerInterface;
use Symfony\Component\Security\Http\Authentication\AuthenticationSuccessHandlerInterface;

class AuthenticationHandler implements AuthenticationSuccessHandlerInterface, AuthenticationFailureHandlerInterface
{
    public function __construct(
        private RouterInterface $router
    ) {
    }

    /**
     * @return Response
     */
    public function onAuthenticationSuccess(Request $request, TokenInterface $token)
    {
        // Remove post_logout if set
        $request->getSession()->remove('post_logout');

        $format = $request->request->get('format');

        if ('json' == $format) {
            $array    = ['success' => true];
            $response = new Response(json_encode($array));
            $response->headers->set('Content-Type', 'application/json');

            return $response;
        } else {
            $redirectUrl = $request->getSession()->get('_security.main.target_path', $this->router->generate('mautic_dashboard_index'));

            return new RedirectResponse($redirectUrl);
        }
    }

    /**
     * @return Response
     */
    public function onAuthenticationFailure(Request $request, AuthenticationException $exception)
    {
        // Remove post_logout if set
        $request->getSession()->remove('post_logout');

        $format = $request->request->get('format');

        if ('json' == $format) {
            $array    = ['success' => false, 'message' => $exception->getMessage()];
            $response = new Response(json_encode($array));
            $response->headers->set('Content-Type', 'application/json');

            return $response;
        } else {
            $request->getSession()->set(Security::AUTHENTICATION_ERROR, $exception);

            return new RedirectResponse($this->router->generate('login'));
        }
    }
}