Domain is not secured over WWW even though Let's Encrypt certificate is installed

Symptoms

• Warning in browser is shown whenever the domain with installed Let's Encrypt SSL is accessed with www-prefix like https://www.example.com:

  CONFIG_TEXT: NET::ERR_CERT_AUTHORITY_INVALID

  ---

  CONFIG_TEXT: Your connection is not private
  This server could not prove that it is www.example.com; its security certificate is from example.com. This may be caused by a misconfiguration or an attacker intercepting your connection.

• Certificates is shown "WWW domain not secured" under Domains > example.com >  SSL/TLS Certificates

Cause

The certificate was issued only for the domain example.com, without www-prefix.

Resolution

Reissue certificate with www-prefix:

1. Login to Plesk;
2. Open Domains > example.com > SSL/TLS Certificates
3. Check the option Include a "www" subdomain for the domain and each selected alias
   
   
4. Click Reissue Certificate.