SSL privacy warning is shown when accessing Plesk panel UI using customer’s domain name at https://example.com:8443:
Your connection is not private
Connection not secure
This is expected behaviour. Plesk interface is secured by an SSL issued to the server hostname. Other domains, like example.com, had their own certificates that are issued for a particular domain name. So https://example.com will show valid SSL issued for example.com, while https://example.com:8443 will detect that SSL for example.com is not the same as for the server hostname.
The link may be used by any customer, domain owner, reseller, or administrator.
Alternatively, issue the UCC SSL certificate covering several domains from any convenient SSL authority. After that install this certificate using instructions.