This might be caused by... / #2722

from class_task.inc.php

            new FieldBool(  array('name'=>'is_milestone',
                'title'=>__('is a milestone'),
                'tooltip'=> __('milestones are shown in a different list'),
                'view_in_forms'=>false,
                'default'   =>false,  # this might not work
            )),

But it would be good to reproduce such issues here! Please provide my.ini and php.ini.