This happened to me when I had two instances of VS 2017. The first instance had a default Xamarin project with the old Nuget for Xamarin. Debug debugging on the Mac build server.
Then a second VS instance was launched with the Xamarin project on the latest Nuget for Xamarin. Both instances were connected to the MAC build server. When I tried to build, I would get this error.
Stopped both VS instances and started only one. I uploaded my project using the latest Xamarin Nuget - now everything builds OK.
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