wp-comments-post.php returns blank

I got problem, when visitors try to comment to my site, they got blank wp-comments-post.php.
it seems that the comment doesn’t inserted to database.
i tried to replacing wp-comments-post.php with new one but it didn’t work.
i have no idea about this.
please look at http://www.masbuchin.com and try to comment.
may be someone can help me