I have configured the Control Panel to install my Telerik selection into "V:\Telerik".One entry in that directory is a folder "ToolboxNuGetPackages".
Last night I updated the Telerik libraries to the current version using the Progress Control Panel. A few minutes ago, I created a new solution for a WPF netcoreapp (in Jetbrains Rider), added references to the required Telerik libraries and tried to build the otherwise untouched solution.
It failed during the ResolvePackageAssets task, because nuget was unable to find the fallback package folder "C:\Program Files (x86)\Progress\ToolboxNugetPackages".
With a bit of help from StackOverflow I found that this setting is defined in the file "C:\Program Files (x86)\NuGet\Config\Telerik UI for WPF.config", and I for one did not put it there. In fact, I had changed it a few days ago,while investigating some other build failures; after all, it was a non-existing path, so things could not get worse. Now, after the update, the original path is back ... and as installer the Control Panel is my prime suspect.
So I'd like to ask somebody at Telerik to check if the Control Panel does write the contents of that file and if so, to make sure that it uses the actual location of the ToolboxNuGetPackages folder.
In my case, I changed the path to "V:\Telerik\ToolboxNuGetPackages", unloaded and reloaded the project and it built without further problems.
I tried to submit this via the "Get Technical Support" item in my account page, but the UI is so user-friendly that submissions for the Control Panel are impossible there.