I’ve been seeing a lot of 520 & 521 errors on two different sites with two different hosts. One just came back to me with a reply which makes a lot of sense.
The reason this has suddenly become a problem is due to us seeing a large number of automated bot attacks originating from CloudFlare IPs, for this reason our automatic intrusion system has picked these IPs up and blocked them on our firewalls.
This makes sense. And you can argue that this is a correct response by them. They now say that they have removed CloudFlare IPs from their firewall and are looking for better ways to analyse CloudFlare traffic. But they can’t guarantee it won’t happen again.