Thêm vào file magento.conf

# 16-sept-2012 parametros para evitar el 502
    fastcgi_temp_file_write_size 10m;
    fastcgi_busy_buffers_size 512k;
    fastcgi_buffer_size 512k;
    fastcgi_buffers 16 512k;
    fastcgi_connect_timeout 300;
    fastcgi_send_timeout 300;
    fastcgi_read_timeout 300;
    fastcgi_intercept_errors on;
    fastcgi_next_upstream error invalid_header timeout http_500;