As you can see in one of my earlier posts “the vpn server may be unreachable. (-5)”, the problem can sometimes be caused by some sort of VNC server on the machine.
Other possible problems can be:
- the firewall rules on local machine, or on the network gateway ( I have rarely found this to be the problem with this error)
- problems with the FortiGate device, in most of the time the device would be the problem and the problem would go away after the reboot of the FortiGate device, but would come again after the few days. In this case the problem would most of the time be with the extensive logging of the traffic and the events on the device. So try to remove traffic logging on some of the rules or events.