Possible bug in includes/session.php:266:
$_EXTRA_URL = array();

1. A user clicks a "delete board cookies" link.
2. The user clicks "Yes" to the confirm box.
3. The redirect to the index.php does not contain any $_EXTRA_URL parameters because the session is killed (ucp.php:167) and then initialized (ucp.php:168).

Possible fix:
if ( !isset($_EXTRA_URL) )
$_EXTRA_URL = array();

I don't fully understand the purpose of this $_EXTRA_URL variable so this may not be a bug at all.