After enabling Cloudfare one of my site ( https://onlineleads.fi ) get this kind of errors, after re-enter url its usually working but it comes just like 4-5 times per referesh. There is not anything problem at site, server or anything - Checked logs.
It’s not Cloudflare problem. It’s your server or your site. Investigate by disable Cloudflare Proxy. Turn on Development mode.
Then,
1, Check your server logs.
2, Check if your header is missing response by using this: Fiddler | Web Debugging Proxy and Troubleshooting Solutions
3, Server down or overloaded.
4, Oversize header. Cloudflare has a limit of 8 KB that can be used for headers.
5, Check TCP Idle Timeouts. It much be at least 300
6, Check if Cloudflare IP was blocked by server. Here is the list of Cloudflare IP: https://www.cloudflare.com/ips/
7, Check PHP service and try to restart all services. But should monitor which cause the server down or overloaded.