Devices with iOS 9.0 or higher will (by default) use the HTTP/2 protocol. This protocol is saver and faster. The HTTP/2 protocol is supported as of Windows Server 2016. However, NTLM authentication (also known as Windows Authentication) is not supported by HTTP/2.
Because of this, devices with iOS 9.0 or higher CAN NOT make an HTTPS connection with a Windows Server 2016 with NTLM authentication. Although it is stated that in this situation, the system should fall back on the HTTP/1 protocol, that does not seem to happen. Apple confirms that it is (at this moment) not supported.
A workaround to be able to work with the mobile apps of Exact Synergy Enterprise in such an environment, is to use Basic authentication. This could be achieved by creating a seperate virtual directory for the mobile apps with Basic authentication, and leaving the ‘normal’ Synergy URL on NTLM authentication.
Note: devices running on Android do not have this issue!