Try this

includes ->index.inc.php

  if($_SESSION[$settings['session_prefix'].'usersettings']['thread_order']==0) $thread_order = 1;
  else $thread_order = 0;
 }
elseif(isset($_GET['thread_order']))
 {
  if(isset($_GET['thread_order'])) $thread_order = 1;
  else $thread_order = 0;
 }
else $thread_order = 1; 

Change to

if($_SESSION[$settings['session_prefix'].'usersettings']['thread_order']==0) $thread_order = 0;
  else $thread_order = 1;
 }
elseif(isset($_GET['thread_order']))
 {
  if(isset($_GET['thread_order'])) $thread_order = 0;
  else $thread_order = 1;
 }
else $thread_order = 0;