FortiClient Connection Error: unknown protocol - ONLY on one particular remote LAN

Mac OSX 10.10.3

FortiClient connects fine from 3 different remote LANs but gives "Unknown Protocol" Connection Error on a 4th. Protocol is SSL. All of these connections are Wi-Fi.  All normal internet access on this 4th LAN works as expected. The LAN is using an ASUS RT-N66U router connected to WAN via Zoom cable modem/Time-Warner. I am baffled. should I be looking at router? modem? something else? Thanks for any suggestions.