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