Raspbian - Apache server not accessable over the internet

With the help of grochmal and Julie Pelletier it works now! (see comments section of my question)

I changed:

ports.conf

Listen 8080

<IfModule mod_ssl.c>
        Listen 8443 https
</IfModule>

sites-available/default-ssl.conf

<VirtualHost *:8443>
        ServerAdmin webmaster@localhost
        DocumentRoot /var/www/html
        ServerName server_name_from_hosts_file:8443
        SSLEngine on
        SSLCertificateFile /etc/apache2/ssl/apache.crt
        SSLCertificateKeyFile /etc/apache2/ssl/apache.key
        ServerSignature On
        CustomLog /var/log/apache2/access.log combined
        LogLevel warn
        ErrorLog /var/log/apache2/error.log
</VirtualHost>

apache.conf

ServerName server_name_from_hosts_file

Port forwarding

New port forwarding rules for the Ports 8080 and 8443.

Thank you so much!