A 520 error occurs when the connection started on the origin web server, but that the request was not completed. The most common reason why this would occur is that either a program, cron job, or resource is taking up more resources than it should causing the server not to be able to respond to all requests properly.
Review the Quick Fix Ideas in this Community Tip for troubleshooting suggestions.
This wouldn’t show up in the firewall log as it’s not a block or challenge. You can see some info in error Analytics though, more info on that here → https://support.cloudflare.com/hc/en-us/articles/115003011431-Troubleshooting-Cloudflare-5XX-errors#500error
If you happen to be using DIrectAdmin + Apache, there’s an ongoing topic here → Sometimes a CF 520 error