HTTP load failed (error code: -999 [1:89]) for Task
Error -999 means NSURLErrorCancelled, your request has been canceled before completion. According to the apple developer blog, there are few changes with ATS (likely that your server is using one of legacy crypto facilities that have been removed in iOS 11.) So give a try and implement the connection:didReceiveAuthenticationChallenge: and see if your server raises authentication challenge if yes then here is something we need to handle.
I hope it will help!