It is very strange your computer can not manage login to VPN via forticlient. you try connect to VPN SSL Fortigate and stop at 40% , and come out message error "Unable to establish the VPN connection. The VPN server may be unreachable (-5)".
- If your connection using wireless then change MTU on "Wirelless Network Connection"
Change your interface MTU computer using Register Editor
- Open regedit as an administrator account
- Navigate to HKLM\System\CurrentControlSet\Services\tcpip\parameters\interface\[Choose the interface in the question] (Do this by checking the correct IP address is in the setting under this key for the adapter you are configuring)
- Once your are in the correct key for your interface, right-click and select new DWORD value (32bit)
- Call it MTU
- Change the Hexadecimal value equal to the setting 514
- Close the Register Editor window and restart your computer
Now you able to login VPN SSL Fortigate without any problem.