Joomla Guest usergroup can't access CiviCRM

When using Joomla setting "Guest user group" set to "Guest" in stead of "Public" (to show only some menu items to non-logged in users) access to CiviCRM is not granted even Public and Guest got same rights.

http://forum.civicrm.org/index.php/topic,23622.msg99240.html#msg99240