You may find yourself unable to access WHMCS after enabling Cloudflare. WHMCS uses non traditional ports by default, but Cloudflare support these, along with a few more. Assuming your domain is “example.com”, please try one of the following to access WHMCS: 

http://example.com:2086 or https://example.com:2087

If the above doesn’t work, find clues in the DNS settings of your Cloudflare account, to guess under which record WHMCS is accessed. The corresponding URL might be: