The internal DI container uses a decorator pattern on whatever IControllerFactory you have registered with MVC. In all cases except when you are calling the /sitemap.xml
endpoint, this class simply calls your registered IControllerFactory. If you have a custom one, you should be able to set a breakpoint in the CreateController method to find out why your IControllerFactory is not returning the controller in this case.
If you are not using a custom IControllerFactory, I would venture to guess this has something to do with your routes being misconfigured in a way that it is checking for a non-existent controller named Scripts. You probably need to add a line similar to this near the top of the RouteConfig.cs file to force MVC not to consider this case when resolving your controllers.
routes.IgnoreRoute("Scripts/{*pathInfo}");
If that doesn't work, I would suggest opening an issue at GitHub, as this will probably require some back and fourth communication to resolve. It would go a long way to finding a solution if you could build a demo project that exhibits the problem and either post it at GitHub or zip it and make it available for download.