Definitely in memory hosting type issue. Recently, I encounter the same issue after updating to MVC 5. I looked on various posts and actually found one that ultimately helped me out. Here is how I solved this.
After installing library for Attribute routing, you must have the following in global.ascx file.
AttributeRoutingHttpConfig.RegisterRoutes(GlobalConfiguration.Configuration.Routes);
In AttributeRoutingHttpConfig class, replace the following code:
routes.MapHttpAttributeRoutes();
with
routes.MapHttpAttributeRoutes(cfg =>
{
cfg.InMemory = true;
cfg.AutoGenerateRouteNames = true;
cfg.AddRoutesFromAssemblyOf<ANY_API_Controller>();
});
Here, ANY_API_Controller refers to any of your apicontroller class from your project.