- Plesk Onyx
1) Plesk Panel is not shown as secure while being secured by Let's Encrypt. The following warning or similar to that appears in browser when open https://pleskpanel.example.com:8443
Your connection is not secure
2) Login page is under different from a hostname domain. For example, hostname is webhosting-server01.example.com.
Login domain does not match with a hostname of which Let's Encrypt certificate was issued in Tools & Settings > SSL/TLS Certificates.
Issue another certificate to pleskpanel.example.com domain by following the article "How to secure Plesk login page with Let's Encrypt certificate."
In case it is hosting on the same server go to Domains > pleskpanel.example.com > Let's Encrypt