我解决了这个问题.与 Xamarin 或 android 无关，问题出在服务端.

I solved this probolem. It was not related to Xamarin or android, problem was on the service side.

首先启用外部请求:IIS Express 启用外部请求

然后为您的应用编辑绑定信息:IISExpress 返回 503来自远程机器的错误

An then edit binding information for your app: IISExpress returns a 503 error from remote machines