Technical Note: Error ’Unable to establish the VPN connection. The VPN server may be unreachable. (-5)’ on FortiClient with SSL VPN
This article provides the solution when the error "Unable to establish the VPN connection. The VPN server may be unreachable. (-5)" is obtained in FortiClient trying to connect to the SSL VPN and it is stuck at 40% after upgrading to 5.4.x from 5.2. or earlier.
This error may occur because the default settings for encryption have changed in FortiOS v5.4.
1. On the FortiClient (Windows) workstation, go to Internet Explorer > Options > Advanced.
2. Change the TLS settings to match those settings on the FortiGate.
For example, if TLS 1.1 and TLS 1.2 are enabled on the FortiGate, enable them in Internet Explorer as well.