Still cannot reproduce it, but this code change should fix it...
Mind you: I still do not understand why the $modSettings is not set on your forum. Something is definately wrong there...
Change the file TPsubs.template.php
Lines 651 and 652
Change from
// if no guest access to forum, then no recent topics
if($modSettings['allow_guestAccess'] == '0' && $user_info['is_guest']) {
to
// if no guest access to forum, then no recent topics
if(empty($modSettings['allow_guestAccess']) && $user_info['is_guest']) {