Hi Sven,
you should know, that the Plesk Control Panel uses port 8443, because port 443 is already used by your webserver. The Panel itself doesn't use your webserver ( but it's very own nginx webserver version on the pre-configured port 8443 ), as an
additional webserver, so that changes can be made on your webserver without loosing connection to the users, who are logged in to the panel and so the panel is save against needed restarts of your webserver in case of changes and modifications.
A trick to avoid high costs for a new certificate is to use certificates from StartSSL (
http://startssl.com ). You might get as much free ( subdomain ) certificates, valid for one year and cost free renewable each year, using the certificate categorie "StartSSL Free (Class 1) certificates". Please read more at:
https://www.startssl.com/?app=39
In order to achieve your goal, you might tell your visitors and customers to visit the URL "
https://servername.example.com:8443" ( matching your current hostname! ), while you secure this subdomain with a SSL certificate from StartSSL.
Odin provides as well a KB - article, how to change the port 8443 of your Plesk Control Panel: