This is a known issue with some versions of Visual Studio and credentials managers. This will happen for any private NuGet package source, not just the Telerik NuGet server.
You can however, add a config file to Visual Studio's NuGet package manager to remember your credentials (because that checkbox never does). Here is one blog post that shows you how
, if that doesn't work for you, you can find many other threads discussing this online.
Lance | Tech Support Engineer, Sr.
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