You can quickly determine if this a problem with the server, or a problem with Visual Studio 2015. Open your web browser and do the following:
1 - Enter the URL in browser: https://nuget.telerik.com/nuget
2 - The web browser will prompt you for your credentials
If you successfully connect, you'll be served the WebService details and they'll appear on the page
This means that the problem is with Visual Studio 2015 the NuGet Credential manager for Visual Studio 2015. There is a known issue where sometimes it clips characters from the end of the auth request, so the credentials being sent to our server are wrong, even though you typed in the correct ones.
Try my suggestion to the other folks in this thread, delete the package source and re-add it. Alternatively, if you can, switch to Visual Studio 2017. I have not received a report of this happening in VS 2017.
Let us know how it goes.
Lance | Tech Support Engineer, Sr.
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