I had the same problem and found a solution. The problem for me was the tweetmeme module. I uninstalled it and the error was gone.
If you don't wanna uninstall it, you can use a simple trick to deactivate it in the rss.php:
rss.php:
<?php
// The source code packaged with this file is Free Software, Copyright (C) 2005 by
// Ricardo Galli <gallir at uib dot es>.
// It's licensed under the AFFERO GENERAL PUBLIC LICENSE unless stated otherwise.
// You can get copies of the licenses here:
// http://www.affero.org/oagpl.html
// AFFERO GENERAL PUBLIC LICENSE is also included in the file called "COPYING".
include_once('Smarty.class.php');
$main_smarty = new
Smarty;
$rss = 'rss';
modules/tweetmeme/init_tweetmeme.php:
<?php
if(defined('mnminclude') and $rss != 'rss'){
...
So it checks, if it's the rss.php, or not.
Best Regards
vardump