An incompatibility between mymenu -a library for module developpers- and XoopsCube 2.1x was found.
If you are a module developper and use "mymenu" library, update include/preferences.inc.php from the latest mymenu archive.
If you are an user for Xoops Cube 2.1x and want to use a module using mymenu right now, open include/preferences.inc.php inside the module by text editor.
Insert this line just after the line of "<?php".
$config_handler =& xoops_gethandler('config');