Port 8443 is used in practice as an alternate HTTPS port (IANA officially registers it for something else). Well-known users: the Plesk panel (https://server:8443, HTTP on 8880), Tomcat with TLS, UniFi Network and many appliances.
Admin interfaces should not be open to everyone. Restrict to admin IPs or VPN where possible; Plesk offers IP access restriction under Tools & Settings. Enable 2FA as well.
Typical: certificate warning on 8443 because the panel still uses the self-signed default – assign a Let's Encrypt certificate to the server hostname in Plesk – or timeouts because the hosting firewall doesn't allow 8443.
Check
curl -kI https://server.example.com:8443