Closed Unassigned: ModalPopupExtender error [27771]

I am getting the following error after updating to 15.1

Sys.WebForms.PageRequestManagerServerErrorException: Extender control 'GeneralModalPopupExtender' cannot extend 'ClientEditBtnModalPopupHidden'. Extender controls of type 'AjaxControlToolkit.ModalPopupExtender' cannot extend controls of type 'System.Web.UI.HtmlControls.HtmlInputHidden'

Any thoughts on this?