It appears there is a bug in the AJAX Control toolkit ModalPupExtender. Somewhere in the prerendered JS there is a semicolon missing when using a ValidationSummary in a ModalPopup.

[ToolboxData("<{0}:AjaxValidationSummary runat=server>“)]
    public class AjaxValidationSummary : ValidationSummary
    {
        protected override void OnPreRender(EventArgs e)
        {
            base.OnPreRender(e);
            ScriptManager.RegisterStartupScript(this.Page,      this.Page.GetType(), this.ClientID, “;”, true);
        }
    }