520 error on website being accessed by a long URL

We’re using the free service and we have a REST service we’re accessing through an nginx server using a querystring string that is really long, ~6800 characters. The request does not appear to make it through to our service, it returns a 520 error from Cloudflare.

This could be an nginx issue however I’m not seeing any errors in those logs. Am I correct in thinking that the URL limit for Cloudflare, including query string, is 16k?

Easy way to check, what happens if you bypass Cloudflare by grey-clouding the DNS entry or using the “pause Cloudflare” option?

(wait at least 5 minutes before testing)

Hmm, the website uses SSL using the certs from CF so I created another server section for port 80 and verified I could hit that directly using curl. I repeated the request again, this time using http rather than https and got the same error.