Domain opens different websites via HTTP and HTTPS

Symptoms

When navigating to a domain name via HTTPS, a different website is displayed, while via HTTP, the correct website opens or is automatically redirected to the HTTPS version.

Causes

The problematic website lacks an SSL certificate, even though there is at least one website with certificate on the web server.

Solution

Connect an SSL certificate to the website: a purchased one, a free one from Let's Encrypt, or a self-signed one.