[Lynne]

Try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit includes/config.php and add this line right under <?php

PHP Code:

define('DISABLE_HOOKS', true); 


You will then need to clear the cache to make sure the forum block updates (there is a cache time for each block and it won't update if you don't make it).