I was getting this error as well, but on actual devices rather than the simulator. We noticed the error when accessing our heroku backend on HTTPS (gunicorn server), and doing POSTS with large bodys (anything over 64Kb). We use HTTP Basic Auth for authentication, and noticed the error was resolved by NOT using the didReceiveChallenge: delegate method on NSURLSession, but rather baking in the Authentication into the original request header via adding Authentiation: Basic <Base64Encoded UserName:Password>. This prevents the necessary 401 to trigger the didReceiveChallenge: delegate message, and the subsequent network connection lost.

我也有同样的问题。解决方案很简单，我已经设置HTTPBody，但还没有设置HTTPMethod POST。修好之后，一切都好了。

我的问题在服务器上。我正在使用Python的BaseHTTPRequestHandler类，我没有在响应中发送正文。当我像下面这样放一个空身体时，我的问题就解决了。

def do_POST(self):
    content_len = int(self.headers.get('Content-Length'))
    post_body = self.rfile.read(content_len)
    msg_string = post_body.decode("utf-8")
    msg_json = json.loads(msg_string)
    self.send_response(200)
    self.end_headers() #this and the following lines were missing
    self.wfile.write(b'') 

我们有这个确切的错误，它原来是NSURLRequest的底层HTTP实现的问题:

据我们所知，当iOS 8/9/10/11接收到一个带有keep - alive头的HTTP响应时，它会保留这个连接以供以后重用(这是应该的)，但它会保留它的时间超过keep - alive头的超时参数(它似乎总是保持连接活跃30秒)。 然后，当应用程序在不到30秒后发送第二个请求时，它会尝试重新使用可能已经被服务器丢弃的连接(如果超过了真正的Keep-Alive时间)。

以下是我们目前找到的解决方案:

Increase the timeout parameter of the server above 30 seconds. It looks like iOS is always behaving as if the server will keep the connection open for 30 seconds regardless of the value provided in the Keep-Alive header. (This can be done for Apache by setting the KeepAliveTimeout option. You can simply disable the keep alive mechanism for iOS clients based on the User-Agent of your app (e.g. for Apache: BrowserMatch "iOS 8\." nokeepalive in the mod file setenvif.conf) If you don't have access to the server, you can try sending your requests with a Connection: close header: this will tell the server to drop the connection immediately and to respond without any keep alive headers. BUT at the moment, NSURLSession seems to override the Connection header when the requests are sent (we didn't test this solution extensively as we can tweak the Apache configuration)

On top of all the answers i found one nice solution. Actually The issue related to network connection fail for iOS 12 onword is because there is a bug in the iOS 12.0 onword. And it Yet to resolved. I had gone through the git hub community for AFNetworking related issue when app came from background and tries to do network call and fails on connection establish. I spend 3 days on this and tries many things to get to the root cause for this and found nothing. Finally i got some light in the dark when i red this blog https://github.com/AFNetworking/AFNetworking/issues/4279

据说iOS 12系统有漏洞。基本上，如果应用程序不在前台，你就不能期望网络调用完成。由于这个错误，网络呼叫被中断，我们在日志中得到网络故障。

我给你的最好的建议是，当你的应用从后台到前台有网络呼叫时，提供一些延迟。使调度中的网络调用具有一定的延迟。你永远不会得到网络呼叫掉线或连接丢失。

不要等待苹果在iOS 12中解决这个问题，因为它仍然没有修复。 你可以通过为你的网络请求NSURLConnection, NSURLSession或AFNetworking或ALAMOFIRE提供一些延迟来解决这个问题。欢呼:)

我也面临着同样的问题， 我已经为应用程序的慢速网络测试启用了网络链接调节器。这有时会产生这个错误， 当我从设置>开发>网络链接调节器中禁用它时，它解决了我的问题。

希望这能帮助到一些人。