dotnet restore task suddenly started failing (The process cannot access the file)

The process cannot access the file 'C:\Program Files\dotnet\sdk\NuGetFallbackFolder\microsoft.aspnetcore.razor.design\2.1.1\tools\
Microsoft.AspNetCore.Razor.Language.dll' because it is being used by another process.)) 
(The process cannot access the file 
'C:\Program Files\dotnet\sdk\NuGetFallbackFolder\microsoft.aspnetcore.razor.design\2.1.1\tools\Microsoft.AspNetCore.Razor.Language.dll' 
because it is being used by another process.) 
---> System.IO.IOException: The process cannot access the file 'C:\Program Files\dotnet\sdk\NuGetFallbackFolder\microsoft.aspnetcore.razor.design\2.1.1\
tools\Microsoft.AspNetCore.Razor.Language.dll' because it is being used by another process.