Yup, the error log is enabled, but not much information show.
The most relevant is this
2016/07/14 17:06:44 [wa] 4995#4995: *487 an upstream response is buffered to a temporary file /var/cache/nginx/fastcgi_temp/6/03/0000000036 while reading upstream, client: 10.109.248.181, server: www.hermes-ottoint.com, request: "GET /productinfo/witt-weiden/wp-admin/upload.php HTTP/1.1", upstream: "fastcgi://unix:/var/run/php-fpm/php-fpm-scarp01.sock:", host: "www.hermes-ottoint.com", referrer: "http://www.hermes-ottoint.com/productinfo/witt-weiden/wp-admin/media-new.php"
My server is within intranet able to access Inteet but unable to being access from public. Is it the reason behind? Is there any response or connection is needed from S3 back to my server?