May I ask you to share what is the error code and/or a screenshot of it?
Sounds like your origin host/server needs either more time to execute the request than Cloudflare’s default 100s?
May I ask if it is the below error or some other?
May I ask if your Website is working over secured HTTPS connection?
If so, does your app work over a compatible and supported port with Cloudflare proxy mode 
?:
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 … )?
Could you re-check if Cloudflare is allowed to connect to your origin host to as follows in the below article:
Nevertheless, Cloudflare IP addresses list can be found here: