Cloudflare is a very useful add on for a site.

It provides a cache for the contents of a site reducing its load and also it’s a CDN that helps a site to be accessible quickly from all over the world.
Unfortunately there are some side effects. From the moment you change the nameservers cpanel is not accessible any more.
The cpanel subdomain must also be included when cloudflare is configured in order to avoid this problem.
The same must be done for the mail subdomains and any other subdomains that have been defined.