mautic.NOTICE: PHP Notice - Trying to access array offset on value of type bool

Hi to all! I’m new to Mautic and got trouble in the setup. I hope somebody can help me with this issue. Thank you so much in advance!

My Mautic version is: 3.3.4
My PHP version is: 7.4.23
My Database type and version is: MySQL 8.0.26

My problem is:
I added a email (successful after testing the connection) in the bounce section in the configuration in Mautic. Then I got this screen and error:

The site is currently offline due to encountering an error. If the problem persists, please contact the system administrator.

These errors are showing in the log:

[2021-09-26 08:48:45] mautic.NOTICE: PHP Notice - Trying to access array offset on value of type bool - in file /home/xxx/domains/mautic.xxx.com/public_html/app/bundles/CoreBundle/Controller/ExceptionController.php - at line 120 {“request”:"[object] (Symfony\Component\HttpFoundation\Request: GET ///remote/fgt_lang?lang=/…/…/…/…//////////dev/ HTTP/1.1\r\nAccept: /\r\nAccept-Encoding: gzip, deflate\r\nConnection: keep-alive\r\nHost: 161.97.103.19\r\nUser-Agent: python-requests/2.6.0 CPython/2.7.5 Linux/3.10.0-1160.41.1.el7.x86_64\r\nX-Php-Ob-Level: 1\r\n\r\n)",“exception”:"[object] (Symfony\Component\Debug\Exception\FlattenException: {})",“logger”:null,“class”:“Symfony\Component\HttpKernel\Exception\NotFoundHttpException”,“currentContent”:"",“layout”:“Error”,“code”:404,“anonymous”:true,“baseTemplate”:“MauticCoreBundle:Default:slim.html.php”,“templatePage”:false,“template”:“MauticCoreBundle:Error:404.html.php”,“templating”:"[object] (Symfony\Bundle\FrameworkBundle\Templating\DelegatingEngine: {})",“statusText”:“Not Found”,“url”:"///remote/fgt_lang?lang=/…/…/…/…//////////dev/",“urlParts”:false}