I'm using DayPilotPro (3.8.1086) and as soon as I add ModalPopupExtender from the AjaxControlToolkit to the page it gives this runtime error: "Extender controls may not be registered before PreRender". The ModalPopup works fine in any of the other pages that do not use DayPilot.

All pages inherit from BasePage and use a (the same) master page. There are no overrides on (page) render events in my code. Any ideas what may cause this?