The NSURLErrorDomain -999 (and sometimes -1003, -1004, or -1200) message happens semi-frequently during Duo pushes. The error has only been reported on iOS devices. The error happens when there is a network error within the app during a push. A troubleshooting step to resolve the issue is to toggle wifi and use cellular data, or to switch to a different wifi network if available.

Duo's Documentation: https://help.duo.com/s/article/4010?language=en_US