How to understand what can cause the cloudflare 521 error?

On the website with the cloudflare error 521. Judging by the description, it is a bug when nginx resets the connection. As you can tell that he doesn't like? In what direction to dig? In the error logs of the crime can not see.
March 19th 20 at 09:15
1 answer
March 19th 20 at 09:17
As he says cloudfire, the problem is that 1. Apache or nginx lies. That is to start checking with that.
2. The web server block request from claudere, that is, to connect provider:

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