Tweak fastcgi buffers, just in case.

[artmouse]

fastcgi_max_temp_file_size 0;
fastcgi_buffer_size 128k;
fastcgi_buffers 256 4k;
fastcgi_busy_buffers_size 256k;
fastcgi_temp_file_write_size 256k;

[magenx]

you have to read this page http://nginx.org/en/docs/http/ngx_http_fastcgi_module.html
and this page https://gist.github.com/magnetikonline/11312172to understand how to configure these buffers.