This problem is back with the latest version of Telerik UI R2 2021 (2021.1.119).
After updating the packages using the Telerik Control panel no projects will build om Visual Studio 2019 and Nuget packages will not install.
C:\Program Files\dotnet\sdk\5.0.102\NuGet.targets(131,5): error : The local source 'C:\Program Files (x86)\Progress\ToolboxNuGetPackages' doesn't exist.
The workaround is to manually create this folder: 'C:\Program Files (x86)\Progress\ToolboxNuGetPackages'