[c#] 405 method not allowed Web API

Another possible issue which causes the same behavior is the default parameters in the routing. In my case the controller was located and instantiated correctly, but the POST was blocked because of default Get action specified:

config.Routes.MapHttpRoute(
    name: "GetAllRoute",
    routeTemplate: "api/{controller}.{ext}"/*,
    defaults: new { action = "Get" }*/ // this was causing the issue
);