That is most definitely a Let's Encrypt server attempting to validate an http-01 challenge. Keep in mind that the certbot nginx authenticator adds an exception to your nginx configuration to serve the challenge file. Since you already have a similar exception, there might be a conflict. Additionally, your webroot directory (root) for port 80 is only defined inside of your current exception, which might cause a failure to serve the challenge file if certbot's exception interferes. To avoid this issue entirely, we can switch back to the webroot authenticator, but include the correct webroot this time in the certbot command.
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