namespace DiskViewBox
{
    [Serializable] //<- DiskViewItem als serialisierbar markieren löst dein Problem
    public class DiskViewItem
    {
        protected Point position;

        public Point Position
        {
            get { return position; }
            set { position = value; }
        }
    }
}

[System.ComponentModel.DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
public List<DiskViewItem> Items {
	get { return items; }
	set { items = value; }
}