It only shows up when i choose in the last step something other then “sofort”. With the option “sofort” it works fine.
Following the contentn of the debug logfile and can send you some login data over an non public channel.
[11-Sep-2023 09:51:06 UTC] PHP Fatal error: Cannot declare class PHPMailer\PHPMailer\PHPMailer, because the name is already in use in /var/www/vhosts/bernhard-goldkuhle.com/2023.bernhard-goldkuhle.com/wp-includes/PHPMailer/PHPMailer.php on line 32
I deactivated the Akeeba Backup Plugin temporarily and tried it again. It worked and indicated me another mistake i made in the form itself. After the fix of the new discovered mistake, i reactivated Akeeba Backup and now everything works fine together.