Symptoms
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A domain that exist in Plesk (e.g. example.com) and is secured with an SSL certificate is used for Plesk URL.
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When opening Plesk using this URL (https://example.com), the page shows an SSL warning.
Cause
Plesk uses an SSL certificate configured at Tools & Settings > SSL/TLS Certificates.
Answer
Secure Plesk interface with a free Let's Encrypt certificate using the instructions from this KB article.