Could not load assembly System.Threading.Tasks.Extensions

Everything was fine. Then can no longer build. I was trying to add plugin.multilingual to my project when this error came up. Now nothing I do can fix it. I removed the plugin, went back to the previously checked in version of my app, rebooted my PC (getting desperate), deleted the packages folder... Nothing works, project is just dead now.

Error       Exception while loading assemblies: System.IO.FileNotFoundException: Could not load assembly 'System.Threading.Tasks.Extensions, Version=4.1.1.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'. Perhaps it doesn't exist in the Mono for Android profile?

File name: 'System.Threading.Tasks.Extensions.dll'

   at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.Resolve(AssemblyNameReference reference, ReaderParameters parameters)
   at Xamarin.Android.Tasks.ResolveAssemblies.AddAssemblyReferences(DirectoryAssemblyResolver resolver, ICollection`1 assemblies, AssemblyDefinition assembly, Boolean topLevel)
   at Xamarin.Android.Tasks.ResolveAssemblies.Execute(DirectoryAssemblyResolver resolver)   TyreSense.Android           

Version 4.1.1.0 doesn't even seem to exist on the nuget page for System.Threading.Tasks.Extensions. Cannot find any reference to that version in my project either. Any ideas how to fix this?

I have the very latest VS 2017 professional as of today.