I would bet WordPress due to PHP limits, or like if your server cannot handle a lot of traffic (if that is the case maybe?).
Other way, it could be some Firewall issue, or like your origin host / server is somehow limiting and/or not allowing Cloudflare IP addresses to connect to it.
Furthermore, a 520 error is also know to happen due to wrongly or incorrectly configured SSL.
Kindly, may I suggest looking for a possible solution into my post from below (or at least a troubleshooting) which could help you out as far as it contains all the needed: