Unable to establish the VPN connection. The VPN server may be unreachable.

I know this is a common error, but I' m having a hard time tracking down the cause for our particular setup. I' ve tested it on a variety of different OS' s (Vista, 7,8) and a few versions of the Forticlient program, but we always get this result when connecting from any PC recently. The setup had been working fine previously, though we had seen this error before a couple of times (normally a reboot sorted it out). I know that Teamviewer and other VNC clients are known to cause issues, but I' ve been testing this on a machine with no clients installed and windows firewall disabled and still get the same error. What makes it stranger is that we can consistently connect to our Fortigate VPN when using the iPad' s inbuilt VPN software. Any assistance you could give would be much appreciated! Thanks!