Domain example.com is secured by valid certificate. Why is SSL privacy warning shown when accessing Plesk panel UI using customer's domain name at https://example.com:8443?
Your connection is not private
Connection not secure
Plesk interface on port 8443 is different service and is secured by an SSL issued to the server hostname. Domain example.com at default pot 443 has its own certificate issued for this 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.
To avoid the privacy warning, access Plesk using main server hostname URL secured by Let's Encrypt. For example, if the hostname is hostname.com, the link will be https://hostname.com:8443.
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.
How to configure a redirect from the domain name/IP address to the server hostname on port 8443 in Plesk for Linux?
How to use different SSL certificates for Plesk UI (over port 8443)?