Hey guys I have an answer: when writing headers in wp-includes/feed-rss2.php, Wordpress adds automatically 5 lines before the XML begins, producing a non-standard feed.
Just modify this line to make it work fine:
----from
header('Content-Type: ' . feed_content_type('rss-http') . '; charset=' . get_option('blog_charset'), true);
----to
header('Content-Type:'.feed_content_type('rss-http').';charset='.get_option('blog_charset'),true);
if it doesn't work, try commenting the line.
