Connection to Plesk UI is displayed as insecure despite having a valid Let’s Encrypt certificate installed

Symptoms

• Connection to Plesk UI is displayed as insecure despite having a valid Let's Encrypt certificate installed.
  The similar warning appears in the browser when opening https://plesk.example.com:8443/:

  Your connection is not secure

• The certificate was issued for the server's hostname, for example, the hostname is webhosting-server01.example.com:

  # echo 'Q' | openssl s_client -connect plesk.example.com:8443 -showcerts 2>&1 | grep =/CN=
  subject=/CN=webhosting-server01.example.com

Cause

The domain name used to access Plesk UI does not match with the certificate's Common Name.

Resolution

• Log in using the server's hostname: https://webhosting-server01.example.com:8443.

• Change the server's hostname in Tools & Settings > Server Settings > Full hostname and issue a new certificate for it according to the article: How to install or renew SSL certificate for Plesk login page?

Note: Currently it is not possible to secure Plesk UI with more than one certificate: How to use different SSL certificates for Plesk UI (over port 8443)?