A website hosted in Plesk is not accessible with HSTS enabled: Potential Security Issue Your connection is not private

Symptoms

A website with HTTP Strict Transport Security (HSTS) enabled at Domains > example.com > SSL/TLS Certificates is not accessible with the following error message:

• in Firefox:

  PLESK_INFO: Did Not Connect: Potential Security Issue
  Firefox detected an issue and did not continue to example.com. The website is either misconfigured or your computer clock is set to the wrong time.
  It’s likely the website’s certificate is expired, which prevents Firefox from connecting securely.
  
  What can you do about it?
  example.com has a security policy called HTTP Strict Transport Security (HSTS), which means that Firefox can only connect to it securely. You can’t add an exception to visit this site.

  The issue is most likely with the website, and there is nothing you can do to resolve it. You can notify the website’s administrator about the problem.

• in Google Chrome and Microsoft Edge:

  PLESK_INFO: Your connection is not private
  Attackers might be trying to steal your information from example.com (for example, passwords, messages, or credit cards). Learn more
  NET::ERR_CERT_AUTHORITY_INVALID
  ...
  You cannot visit example.com right now because the website uses HSTS. Network errors and attacks are usually temporary, so this page will probably work later.

Cause

The domain has a not-trusted self-signed SSL certificate assigned to secure HTTPS connection. When HTTP Strict Transport Security (HSTS) is enabled and an SSL certificate is invalid, the website will become unavailable.

Another reason is that the website is Disabled.

Resolution

Apply one of the following options:

• (Recommended) Install a valid SSL certificate (for example, a free Let's Encrypt certificate).

• Disable HTTP Strict Transport Security (HSTS) for the domain.

• Enable the website in Domains > example.com > Activate.