Hello Ron, 

I am sorry to hear about the issue you are experiencing. 

Indeed, I was able to reproduce it even when the installation is on C, but after I was hit once the same problem. As a workaround on my side helped the following: 

1. Removing the nugget from the project

2. Clearing NugetManager cache  (from Tools => Nuget Package Manager =>Package Manager Settings =>click Clear All Nuget Cache(s))

3. Restarting the computer

4. Adding the missing folder: C:\Program Files (x86)\Progress\Telerik UI for WPF R2 2020\ToolboxNugetPackages

5. Adding the new nuget package

Please give this a try and let me know if this works for you.

Also could you please give me more detailed steps to reproduce: you open an existing project (NetCore, with NoXaml binaries?), using Visual Studio (2019, 2019 Preview?), have you open the ToolBox after the new installation of Telerik UI for WPF in VS 2019 Preview and before opening the project; how you upgrade to the new version - using the Telerik NuGet server or by using a local package source?

Thank you in advance for your cooperation.

Regards,
Milena
Progress Telerik