NGINX 502 caused by max_buffer_size

Error log:
2018/07/12 08:49:42 [error] 20495#20495: *153 upstream sent too big header while reading response header from upstream, client: 113.178.33.148, server: yoursite.com, request: "POST /core/aha.php HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "timesheet.powergatevn.com", referrer: "http://yoursite.com/core/kkk.php"

https://www.digitalocean.com/community/tutorials/understanding-the-nginx-configuration-file-structure-and-configuration-contexts
https://www.scalescale.com/tips/nginx/502-bad-gateway-error-using-nginx/
http://nginx.org/en/docs/http/ngx_http_fastcgi_module.html#fastcgi_buffer_size

https://stackoverflow.com/questions/23844761/upstream-sent-too-big-header-while-reading-response-header-from-upstream#
https://www.digitalocean.com/community/tutorials/how-to-configure-logging-and-log-rotation-in-nginx-on-an-ubuntu-vps

server {
        listen 80;
        listen [::]:80;
             
        root /var/www/your_site;
        index index.html index.php;
        fastcgi_buffers 8 16k;
        fastcgi_buffer_size 32k;
             
        access_log  off;
        #error_log off;
        error_log  /var/log/nginx/error.log; # tempory turn on log
             
        server_name test.yoursite.com www.test.yoursite.com;

        location / {
                try_files $uri $uri/ =404;
             
                location ~* \.(?:ico|css|js|gif|jpe?g|png)$ {
                    access_log        off;
                    log_not_found     off; 
                    expires 360d;
                    add_header Pragma public;
                    add_header Cache-Control "public";
                }     
        }
               
        location ~ \.php$ {
            fastcgi_pass 127.0.0.1:9000;
            #fastcgi_pass unix:/run/php/php7.0-fpm.sock;
            include snippets/fastcgi-php.conf;
        }     
                 
}

Try test config: $ sudo nginx -t
Then reload (or restart) nginx ? $ sudo service nginx reload  [or restart]