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>>0x800a139e - Microsoft JScript runtime error: ASP.NET Ajax client-side framework failed to load.
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>Could someone - who knows - explain to me, what specifically is "ASP.NET Ajax client-side framework"? Is it a particular .js file? if so, which one?

It's a whole bunch of files. If you use Nuget Package Manager and Download Microsoft.AspNet.ScriptManager.MSAjax you'll get 11 .js files. Try it in a new, empty web forms application and it adds these to a Scripts/WebForms/MSAjax folder.