copy this code in a php file, copy this file in your plugins-folder and after this activate it on the backend of WordPress. I hope this helps, but i dont test it, write it on scratch.
<?php
/**
* Plugin Name: Non-UTF characters in RSS feed
* Plugin URI: http://wordpress.stackexchange.com/questions/37845/non-utf-characters-break-rss-feed/
* Description: Filter content for unicode characters
* Version: 1.0.0
* Author: Frank Bültge
* Author URI: http://bueltge.de
* License: GPLv3
*/
// This file is not called from WordPress. We don't like that.
! defined( 'ABSPATH' ) and exit;
foreach ( array( 'the_content_rss', 'the_excerpt_rss', 'the_title_rss', 'comment_text_rss' ) as $filter )
remove_filter( $filter, 'filter_non_utf8_chars', 0 );
function filter_non_utf8_chars( $content ) {
return htmlentities2( $content );
}
?>