调用HTTPS负载均衡器时SignalRHub出现CORS跨域错误求助

项目基于AngularJS 1.8.2与.NET服务器上的SignalR Hub 2.4.3开发，刚完成从HTTP到HTTPS的迁移。其中一个环境包含两台服务器：一台普通HTTP服务器，一台HTTPS负载均衡器。由于HTTP不安全，我们需要通过负载均衡器访问SignalR Hub，但协商阶段触发了以下CORS错误：

从源'https://stg.fa.lda.com'访问'https://stglb.bo.lda.com/UsersHub/signalr/negotiate?clientProtocol=1.5&AppSourceID=240&connectionData=%5Bname%22userhub%22&_=1713'的XMLHttpRequest已被CORS策略阻止：请求的资源上不存在'Access-Control-Allow-Origin'标头。

其他直接调用普通HTTPS服务器的环境中，协商过程是正常的。我查找了相关解决方案，但大多涉及IConfigureServices（本项目未使用该方式），且很少提及负载均衡的场景。

以下是当前的配置代码：

Startup类配置 #

public class Startup
{
  public void Configuration(IAppBuilder app)
  {
    var idProvider = new CustomUserIdProvider();

    GlobalHost.DependencyResolver.Register(typeof(IUserIdProvider), () => idProvider);

    // Branch the pipeline here for requests that start with "/signalr"
    app.Map("/signalr", map =>
    {
        // Setup the CORS middleware to run before SignalR.
        // By default this will allow all origins. You can
        // configure the set of origins and/or http verbs by
        // providing a cors options with a different policy.
        map.UseCors(CorsOptions.AllowAll);
        var hubConfiguration = new HubConfiguration
        {
            // You can enable JSONP by uncommenting line below.
            // JSONP requests are insecure but some older browsers (and some
            // versions of IE) require JSONP to work cross domain
            // EnableJSONP = true
        };
        // Run the SignalR pipeline. We're not using MapSignalR
        // since this branch already runs under the "/signalr"
        // path.
        map.RunSignalR(hubConfiguration);
    });
   }
 }

WebApi配置 #

public static class WebApiConfig
  {
   public static void Register(HttpConfiguration config)
    {
    // Web API configuration and services

    // Web API routes
    config.MapHttpAttributeRoutes();

    config.Routes.MapHttpRoute(
        name: "DefaultApi",
        routeTemplate: "api/{controller}/{id}",
        defaults: new { id = RouteParameter.Optional }
    );
  }
}

已经配置了允许所有源的CORS，但问题仍未解决。请问针对负载均衡器场景有没有特定的配置方案？

内容的提问来源于stack exchange，提问作者Waldener