Plesk domain behind Cloudflare is not accessible: Invalid SSL certificate Error code 526

Symptoms

Plesk domain is not accessible in a browser with:

CONFIG_TEXT: Invalid SSL certificate Error code 526
Visit cloudflare.com for more information.

What happened?
The origin web server does not have a valid SSL certificate.

What can I do?
If you're a visitor of this website:
Please try again in a few minutes.
If you're the owner of this website:
The SSL certificate presented by the server did not pass validation. This could indicate an expired SSL certificate or a certificate that does not include the requested domain name. Please contact your hosting provider to ensure that an up-to-date and valid SSL certificate issued by a Certificate Authority is configured for this domain name on the origin server. Additional troubleshooting information here.

Cause

The domain is not secured with any SSL certificate. The Domains > example.com > Hosting & DNS tab > Hosting Settings > Certificates drop-down menu is set to None.

Resolution

1. Log into Plesk.
2. If a certificate has been issued recently, select the required certificate in Domains > example.com > Hosting & DNS tab > Hosting Settings > Certificates drop-down menu and apply changes.
3. If the domain doesn't have any certificate yet, issue it as per the article.