If you are using engintron and cloudflare you will encounter some error codes 100x, this is happening because we didn't tell engintron the used shared ip address for our website .
In my case my main server ip were the ip used in all cases direct && cloudflare websites .
I did the following to get it working properly without 100x error codes on my websites that's using cloudflare proxy :
1) Do the following command on the server shell after replacing x.x.x.x with your shared ip address :
echo 'set $PROXY_DOMAIN_OR_IP "x.x.x.x"' >> /etc/nginx/custom_rules
2) Reload nginx with the new configuration :
service nginx reload