Today I got a phone call from a client who wasn’t able to connect to his VPN, he was constantly getting Unable to establish the VPN connection. The VPN server may be unreachable error when he tried to connect, while everybody else was connecting without a problem.
His FortiClient status would always stop at 40 when connecting.
There can be several things that can cause this problem, one of them might be an improper TeamViewer upgrade.
Besides TeamVIewer you might also check if you’re using some other VNC based remote access software on the problematic PC, try disabling it or uninstalling to check if it will get your FortiClient working (I had problems with computers that had TeamViewer and Gbridge installed at the same time).
Uninstall your old version of TeamViewer when upgrading
If you are upgrading your old version of TeamViewer be sure to uninstall the old one before you install the new version, otherwise you will have both versions installed and problems with your FortiClient.
It seems that besides FortiClient, Windows gadgets also wont work properly if you have two versions of TeamViewer 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