When running my app on iOS 11 beta 5 built with Xcode 9 I see this error from several of our network calls.
"NSURLError * domain: @“NSURLErrorDomain” - code: 18446744073709550617"
I've never come across this error before and we haven't made any change to the app currently. For networking, we are using AFNetworking v2.5
So it turns out it was ssl related. Eventually what I did was add an exception for the domain in info.plist and was able to get a reasonable error that said there was an ssl issue. Investigating that showed our cert was weakly signed. We replaced it which resolved the issue.