After installing the Forticlient locally in your machines when you try to connect to other private network it connected through a FortiGate.
This error (-5) happens because these clients still use old Internet Explores. See the below figure and check it with your windows OS.
Some of the Internet Explorer versions need to Enable TLS manually. But some are enable by default. So if you got this error when you connecting to VPN you should enable TLS. You can find them,
Network and Sharing Center -> Internet Options - >
Then try to connect to VPN , it should work.