For some reasons my Telerik credentials for the private NuGet feed in Visual Studio were not working anymore. Eventually a recent reinstall of Visual Studio was to blame. Clearing the NuGet Storage in Options/NuGet Package Manager/General was not helping. Re-Installing Telerik on my device did neither.
Finally I checked the NuGet.Config file, located in C:\Users\my-username\AppData\Roaming\NuGet and saw that no credentials were stored here anymore (but should be).
The below linked Telerik documentation suggested to follow a few steps to setup the credentials via CLI. I placed the provided nuget.exe file in the above mentioned directory and tried the provided PowerShell command. However, this was not working. I had to rewrite the command to:
.\nuget.exe Sources Add -Name "telerik.com" -Source "https://nuget.telerik.com/v3/index.json" -Username "my-email" -Password "my-passwort"
And this finally wrote the credentials successfully to my NuGet.Config file.