I understand. I ignored appveyor.yml in appveyor project settings and changed the Visual Studio image from the dashboard. The project at least starts the build in VS 2013 and then just chokes on new C# features. Changing to VS 2015 throws the above error. This is really looking like an appveyor bug.
When you set ignore appveyor.yml, all your UI settings apply. It must be something in your UI settings which makes AppVeyor fail. To get help in understanding that you can press Export YAML in UI and send results here.