Ok found the problem. It happens if you have bluestork as default, and then try to login with an MC user. Will be fixed in next version, but quickfix now is to replace the _getCurrentAdminTemplate() method in the rt_missioncontrol/lib/rtcore.class.php file with:
function _getCurrentAdminTemplate()
{
$app =& JFactory::getApplication('admin');
return $app->getTemplate();
}