Apple’s iOS 9 breaks a key security method: VPN connections to some corporate servers. Users won’t be able to access some servers over some VPN connections, but other servers will remain available. The last safe iOS version in terms of proper VPN operation is iOS 8.4.1. But iPads, iPhones, and iPod Touches updated to iOS 9 cannot easily be rolled back to prior iOS releases, leaving many users stranded when it comes to reliable VPN access. The bug does not affect the Mac’s OS X operating system, including the golden master version of OS X.”]
Source: https://www.csoonline.com/article/2985024/ios-9-breaks-vpns-prevents-server-access-for-many.html