A warning is shown while accessing Plesk or webmail: The certificate is not trusted or NET::ERR_CERT_AUTHORITY_INVALID

Symptoms

The following error is shown while accessing Plesk or webmail:

• On Google Chrome:

  CONFIG_TEXT: Your connection is not private
  NET::ERR_CERT_AUTHORITY_INVALID

• On Firefox:

  CONFIG_TEXT: This Connection is Untrusted
  (Error code: sec_error_unknown_issuer)
  Certificate Error: The security certificate presented by this website was issued for a different website's address. This problem might indicate an attempt to fool you or intercept any data you send to the server.

• On Internet Explorer:

  CONFIG_TEXT: There is a problem with this website’s security certificate.

• On Opera

  CONFIG_TEXT: Opera cannot verify the identity of the server "XXXX", due to a certificate problem. The server could be trying to trick you

Cause

By default, Plesk is using a self-signed certificate that does not have a signature from the trust center.
Or, the mismatched certificate was applied.

Resolution

Note: If Plesk/WebMail is secured via Let's Encrypt, use hostname/domain name to access it because IP address itself cannot be secured with a valid certificate.

In order to avoid the warning, it is required to have a valid SSL certificate. There are several options on how to achieve that: