Forticlient Error -20101

We have a user  who is getting the error "The VPN server may be unreachable. (-20101) when using the Forticlient SSL VPN client.   A trace route from his computer to our Fortigate is successful i.e. it terminates at our Fortigate.   We have had the "unreachable" error before and have been able to resolve it usually by having the user restart their computer.  It has not worked in this case so we have unistalled and reinstalled the Forticlient software but the error persists.  Does anyone have more information about error code "-20101".