Fortigate 100D FortiOS 6.0.5 SSL VPN being disrupted unusually

[pswolfwind]

Hi,

I am building a remote SSL VPN with an old 100D, but some users are unable to connect this VPN. The failure cases all show this error "Unable to establish the VPN connection, the VPN server maybe unreachable. (-5)" while Forticlient connecting and stops at 40%.

I have tested a few scenarios and have the following results.

 1. failed to connect with company PROD network.

 2. able to connect with DEV network(different ISP)

 3. able to connect to another Fortigate SSL VPN (FortiOS 5.6.3) with the same client and PROD network.

 4. I have asked a few users to try to connect with internet connection from different ISP and some are able to connect.

With those networks that failed to connect, I find that I can't even access HTTPS portal(:443 or :10443). There were certification warning(Fortios factory cert) from Chrome, but unable to proceed to login page( https://A.B.C.D/remote/login?lang=x-sjis ). And the debug log from Forticlient shows that it kept trying to connect but no response.

I can only think of those ISPs are doing application/signature filtering, but then It can't explain the successful case with FortiOS 5.6.3. Is the implementation of SSL VPN varied between ５.6 and 6.0? Has anyone encounter the same symptom?