When you disable the embedded skin for RadDatePicker for ASP.NET AJAX, you need to register manually the skins/images used by the control in order to display them as expected. Review the following online help articles for more details:
(paragraph customizing skins)
In case you merely want to disable or hide the popup button, set the DatePopupButton -> Visible/Enabled
property of the date picker to False
or replace the picker with RadDateInput
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