Programmatic creation of RadScriptManager causes issues with RadAjaxManager
When creating the script manager programmatically, the RadAjaxManager settings do not work.
Creating the script manager programmatically can also cause
RadAjaxManager.GetCurrent(this.Page).ClientID to throw a null reference exception and that code is often use in the markup to reference the ajax manager.
Create the RadAjaxManager programmatically as well.
Note that control creation must be done in Page_Init, but the AjaxSettings must be added in Page_Load.
You can find attached here a basic example.