i also was having the ajax calls that populate the File Lists fail without explanation, until i looked at firebug + firephp – the Ajax call was failing to establish a page session due to Safe Mode being on (and how More Fields establishes a page session; using recursive paths to load wp-load.php:
require_once(dirname(__FILE__).’/../../../wp-load.php’);
not the best way to get ABSPATH and wp-config and wp-settings.
also, ironically, it was throwing this error on pages that shouldn’t even be loading this code (e.g. /wp-admin/edit-pages.php)
hopefully they change how they get ABSPATH in the future to make it more compatible with php/apache/server configurations.