There are a few reasons why this error occurs. The most obvious is your server is down or a certain process is taking too long and your server is very busy. However that may not be the most likely reason for this error especially if this is happening quite frequently. Here are the 2 most likely reasons.
- When your website goes on clouflare, most of the incoming connections to your website are through the cloudflare IPs. If your server does not know about cloudflare IPs, its internal firewall limits access to any connections through those IPs simply because of the number of connections. So it is very important for your server firewall to whitelist those IPs. (Just to tell your server that connections through these IPs are ok). These IPs can be found on the cloudflare site: https://www.cloudflare.com/ips
- You may have tried something which may have triggered some rule set within the clouflare firewall. This block is only limited to you and mostly for a certain time duration. However if you are a developer or are maintaining the website then you need to whitelist your IP by adding it to your IP firewall within the firewall settings on your cloudflare. If you are on a dongle or a network with frequent IP changes then you may need to do this a few times. In that case better to add a range of IP.
For more information visit cloudflare page on this error