Unable to establish the VPN connection
There may be several things that can cause this problem. One of them might be an improper Team viewer upgrade. You might also check if you are using some other VNC based remote access software on the PC.
You should disable it or uninstall to check it will get Forticlient working (I had problems with computers that had Team Viewer and Gbridge installed at the same time).
If you are upgrading from older version to newer version of Team Viewer be sure to uninstall the old one before you install the new version, otherwise you will have both versions installed and problems with your Forti Client.
It seems that besides FortiClient, Windows gadgets also won’t work properly if you have two versions of Team Viewer installed. (Specifically, the default weather gadget in Windows 7 wasn’t getting new data because of it).
With this type of error you might also try checking the router side, if the VPN users are able to connect after the reboot of the FortiGate device, you might try to remove some traffic or event logging on the device.
Its might be due to MS Internet Security & Acceleration firewall . If you want to resolve this issue, try to enable PP Tunneling Protocol from Server Management console.
For more details about the issue see: https://support.microsoft.com/en-us/help/886621/you-receive-an-unable-to-establish-the-vpn-connection-error-message-wh