Where does the 502 error come from, where to dig?

Question

linux

Please tell me where to dig?
There is a chat bot on the VPS server, it is running on node js on localhost, access to it through the nginx proxy.
Everything worked well, but since yesterday there have been problems, at first I thought that the request from facebook (bot in facebook messenger) to my server was slowly passing to my server, it was from facebook on the server, because I debugged the request speed and everything was ok (there is on the screenshot),

then found in the developer panel on facebook, request errors to my server - HTTP 502 Failed to resolve host.
Although it seems like manually curlOM and requests pass in the browser.
I don’t understand where to dig, towards Nginx, DNS or somewhere else.
P.S. In the Nginx error logs /var/log/nginx/error.log, there are only errors that the favicon does not find and only once this
[crit] 4262#4262: *9468 SSL_do_handshake() failed (SSL: error:1417D18C:SSL routines:tls_process_client_hello:version too low) while SSL handshaking, client: 74.82.47.5, server: 0.0.0.0:443

Answer 1

Disabled Nginx, did tried it through ngrok, everything works fine.
How to fix?