A website hosted in Plesk fails to load when Cloudflare is used: ERR_TOO_MANY_REDIRECTS

Symptoms

• A website behind Cloudflare shows the following error message in a web-browser:

  ERR_TOO_MANY_REDIRECTS

  ---

  The page isn’t redirecting properly

• When Cloudflare is disabled, the website loads fine.

• The Permanent SEO-safe 301 redirect from HTTP to HTTPS setting is enabled in Plesk at Domains > example.com > Hosting Settings or custom HTTPS redirection is configured in domain's .htaccess file.

• Trying to issue a Let's Encrypt certificate for the example.com domain fails with the following error:

  Invalid response from https://acme-v02.api.letsencrypt.org/acme/authz-v3/11481922061.
  Details:
  Type: urn:ietf:params:acme:error:connection
  Status: 400
  Detail: Fetching /.well-known/acme-challenge/UcTI6ov0Pc3Fn2IRnNUiFKZKzBu5NaAy4IoscLbYh_I: Redirect loop detected

Cause

The SSL setting is set as "Flexible" in Cloudflare Crypto settings for this domain.

Resolution

Log in to your Cloudflare account > click the domain name > click Crypto and change the SSL setting to "Full (strict)".

Note: The Cloudflare account is created automatically during the initial activation of the extension. Since the Cloudflare extension is enabled on the server, the corresponding account on a Cloudflare side already exists. If the account credentials are forgotten, try to recover the password via the following link to access Cloudflare dashboard: https://www.cloudflare.com/a/forgot-password
The same email should be specified as for the initial extension setup.