Heres the weird part. The error only occurs when clicking a link in the "admins only" block. And the URL the button links to is "pathtowebsite/admin/block.php" but the error page says "pathtowebsite/layout/theme/admin/lists" is not a directory.
What is $_CONF['path'] set to?
What are $_CONF['site_url'] and $_CONF['site_admin_url'] set to?