It seems there is suddenly something out of sync with asp.net core packages.
It looks like Microsoft.AspNetCore.App 2.1.1 was released 16 hours ago on nuget.org but
For bugged SDK 2.1.301 version use: <PackageReference Include="Microsoft.AspNetCore.App" Version="2.1.0" /> and force all other packages 2.1.1 -> 2.1.0.