Frequent, random HTTP 520 errors

Website, Application, Performance
1

What is the name of the domain?

hyperfat.net

What is the error number?

520

What is the error message?

error code: 520

What is the issue you’re encountering

GET hyperfat.net frequently, but randomly, returns HTTP 520

What steps have you taken to resolve the issue?

I’ve checked my web server config and logs, as well as the firewall rules.
The / path on that domain is returning content from the caddy config itself:

respond / 200 {
  body "..."
  close
}

I’ve checked CF settings for this and other domains that are served from the same web server. Other domains don’t experience such an issue.
I followed instructions in the Community Tip - Fixing Error 520

What are the steps to reproduce the issue?

GET hyperfat.net using your favorite HTTP client - try several times if no error.

2

I don’t see a way to attach a HAR file here so I uploaded it to the same server, but the domain is another one and it works without 520s: https://nsauk.in/hyperfat.har

Want to add that the server is IPv6 only and both websites mentioned have IPv4 connectivity only because of CF as a reverse proxy with automatic A-record.

3

An attempt to debug the issue using CF Trace feature ends with the error:
error processing request: can not collect zone trace

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