By default, RadControls for ASP.NET AJAX read and load their scripts from the Telerik.Web.UI.dll, but not from internet. It looks that you have enabled the external CDN script files of RadControls as explained in this article: CDN Overview
That is why my suggestion is to disable the CDN scripts by setting CdnSettings-TelerikCdn="Disabled", e.g.
the Telerik team
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