nginx服务器有时会返回404 Not Found以获取有效的URL

Question

I am using a nginx server with PHP-FPM for web development. While sending continuous AJAX calls from the browser to the server, I sometimes get 404 Not Found errors for a valid URL. (When I open the URL in a new browser tab, it is showing the page properly.)

I am unable to debug why nginx is behaving like this. I don't know if it is dropping connections. What should I do?

I am using default installation of nginx and have not made any changes to it.

This is my nginx.conf

server {

    listen                                  IP address with PORT ssl;
    server_name                             SERVER Name;

    root                                    /u01/projectfolder;

    ssl                             on;
    ssl_certificate                 /etc/nginx/ssl/36287365.net.cert;
    ssl_certificate_key             /etc/nginx/ssl/36287365.net.key;



    index                                   index.php index.html;
    log_not_found                           off;
    charset                                 utf-8;

   location /rainbow {
            try_files $uri $uri/ /rainbow/index.php$is_args$args;
    }

    location ~ \.php$ {
            try_files $uri =404;
            fastcgi_pass   127.0.0.1:9101;
            fastcgi_param   SCRIPT_FILENAME $document_root$fastcgi_script_name;
            include         fastcgi_params;
    }

}

I have also added the log at php side index.php but for 404 requests logs are not generated. I guess the request is not reached to php way fastcgi and nginx.

Please help