Could be something like the Cloudflare IPs aren't allowed in your region for some reason.
Other random possibilities:
Origin web server application crashes
Headers exceeding 16 KB (typically due to too many cookies)
An empty response from the origin web server that lacks an HTTP status code or response body
Missing response headers or origin web server not returning proper HTTP error responses
520 errors are just unknown, so it could be lots of things, most likely on the server end of the website you're going to..
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