SignalR - 发布版本：服务器协商失败

Question

我的 Xamarin 应用程序中遇到一个恼人的错误。该解决方案包含一个 Android 项目和一个可移植类库。在可移植类库中，我有一个类，它使用一种方法与 SignalR 集线器进行通信InitiateConnection

HubConnection conn= new HubConnection(_url);

// Some events will be registrated on the HubConnection here...

if (!string.IsNullOrWhiteSpace(_userName))
{
    conn.Credentials = new System.Net.NetworkCredential(_userName, _password);
}
await conn.Start();

一旦我在调试版本中使用上述方法，它就会正常工作并且我正在接收数据。如果在发布版本中调用此方法，则会引发异常。首先是 Xamarin 未处理的异常。按下继续后，我收到以下异常：

System.InvalidOperationException: Server negotiation failed.


Unhandled Exception:
System.InvalidOperationException: Server negotiation failed.
at Microsoft.AspNet.SignalR.Client.Transports.TransportHelper.<GetNegotiationResponse>b__1 (System.String raw) [0x00000] in <filename unknown>:0 
at Microsoft.AspNet.SignalR.TaskAsyncHelper+<>c__DisplayClass19`2[System.String,Microsoft.AspNet.SignalR.Client.NegotiationResponse].<Then>b__17 (System.Threading.Tasks.Task`1 t) [0x00000] in <filename unknown>:0 
at Microsoft.AspNet.SignalR.TaskAsyncHelper+TaskRunners`2+<>c__DisplayClass3a[System.String,Microsoft.AspNet.SignalR.Client.NegotiationResponse].<RunTask>b__39 (System.Threading.Tasks.Task`1 t) [0x00000] in <filename unknown>:0 
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x00000] in <filename unknown>:0 
at System.Runtime.CompilerServices.TaskAwaiter.GetResult () [0x00000] in <filename unknown>:0 
at Monitoring.Client.MonitorProxy+<InitiateConnection>d__0.MoveNe
[ERROR] FATAL UNHANDLED EXCEPTION: System.InvalidOperationException: Server negotiation failed.
at Microsoft.AspNet.SignalR.Client.Transports.TransportHelper.<GetNegotiationResponse>b__1 (System.String raw) [0x00000] in <filename unknown>:0 
 at Microsoft.AspNet.SignalR.TaskAsyncHelper+<>c__DisplayClass19`2[System.String,Microsoft.AspNet.SignalR.Client.NegotiationResponse].<Then>b__17 (System.Threading.Tasks.Task`1 t) [0x00000] in <filename unknown>:0 
at Microsoft.AspNet.SignalR.TaskAsyncHelper+TaskRunners`2+<>c__DisplayClass3a[System.String,Microsoft.AspNet.SignalR.Client.NegotiationResponse].<RunTask>b__39 (System.Threading.Tasks.Task`1 t) [0x00000] in <filename unknown>:0 
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x00000] in <filename unknown>:0 
at System.Runtime.CompilerServices.TaskAwaiter.GetResult () [0x00000] in <filename unknown>:0 
at Monitoring.Client.MonitorProxy+<InitiateConnectio`

方法的最后一行抛出了异常：await conn.Start();

如果我读取任务的 Exception 属性，我会看到与上面相同的错误。NuGet 包和最近发布的 Xamarin 组件都会出现此问题。有谁知道问题是什么？