Aha, I think I figured it out: it looks like the autoload function of utils.php (supposedly copied from iniset.php) wasn't updated? The preg pattern for Auth_ isn't included.
I seem to be making progress if I update the function autoload in utils.php to:
function __autoload($classname)
{
$filename = preg_replace(
array('/MDB2_(.+)/', '/Mail_(.+)/', '/Net_(.+)/', '/Auth_(.+)/', '/^html_.+/', '/^utf8$/'),
array('MDB2/\\1', 'Mail/\\1', 'Net/\\1', 'Auth/\\1', 'html', 'utf8.class'),
$classname
);
include_once $filename. '.php';
}