This might be a duplicate question. Feel free to simply link solutions. Problem: Customer domain abc.de forwards to www.abc.de and displays xyz.de (my main domain) - while it should forward to https://abc.de, which, if typed manually, displays the correct content. I found that problem, read my SSL might be incorrect and used the certbot command to recreate the certificate of abc.de and updated ISPConfig (which lead to apache2 unable to start because it did not configure the 000-apps.vhost Line 6 ("Listen") correctly - it just left it empty. (This problem I solved by adding my IP: port behind "Listen".) Before writing this post, I found another post stating I should uncheck "LetsEncrypt SSL" in the domain settings of abc.de within ISPConfig, which did not change anything except insecuring the connection.