Thank you for the quick reply. Now I got it working. It seems it was a matter of doing it in the right order:
1. Define the IP address with an IP shown by ifconfig (you can limit it to provide port 443 only).
2. Create site, create SSL certificate (do not use long organisation names, no spcial characters, be patient).
3. Certs must be here, 4 files with the same timestamp: ls -al /var/www/clients/client4/web6/ssl
4. On the Site page, click on SSL and save
5. Check, if the vhost is here: /etc/apache2/sites-available and there is no .err file. The vhost file should have a 443 section.
You should be able to connect via https now.