When you

1. are running under webhost
2. and have System.Web.Http.dll installed in the GAC
3. and the ASP.NET app is configured to run in Medium trust
   

then action methods that return Task<TResult> will cause a MethodAccessException. Specifically, this is an issue with the codegen that we use to invoke those types of methods. One possible solution would be to codegen a call to a public method.