I am afraid this error is coming from your origin host/server and is not related to Cloudflare.
Kindly, check user and group owner rights/permissions over the each directory and the index.php file of /home/httpd/html/index.php. Also check the CHMOD for them.
Otherwise, kindly contact your hosting provider for troubleshooting.
Before moving to Cloudflare, was your Website working over HTTPS connection?
May I ask what SSL option have you got selected under the SSL/TLS tab at Cloudflare dashboard for your domain ( Flexible, Full, Full Strict … )?