Hmm, is your libs/ubbthreads.inc.php file modified at all? Can you take a look at that file at line 2704, and make sure it looks like this:

Code
function generate_rss_feeds($clear='') {

    global $config,$ubbt_lang,$dbh,$html,$smarty;

    if (!is_object($html)) {
        $html = new html;
    } // end if