Имеется аналогичная проблема:
2014/05/03 17:41:45 [error] 1118#0: *2585 upstream prematurely closed connection while reading response header from upstream, client: 112.123.168.55, server: goodwing.ru, request: "POST /wp-comments-post.php HTTP/1.0", upstream: "http://xx.xx.xx.xx:80/wp-comments-post.php", host: "site.ru", referrer: "http://site.ru/2012/10/28/welcome/#comment-17681


Началось после обновления nginx'а до версии nginx/1.5.12