According to Fortinet support, the settings are taken from the Internet options. The Internet Options of the Control Panel can be opened via Internet Explorer (IE), or by calling inetcpl.cpl directly.
Windows Logo + R
Press the Win+R keys enter inetcpl.cpl and click OK.
FortiClient Credential or ssl vpn configuration is wrong. Internet Options Delete personal settings
Select the Advanced tab
Click the Reset… button. If the Reset Internet Explorer settings button does not appear, go to the next step.
Click the Delete personal settings option
Open Internet Options again.
Go back to Advanced tab
Disable use TLS 1.0 (no longer supported)
Add website to Trusted sites
Add the SSL-VPN gateway URL to the Trusted sites. Usually, the SSL VPN gateway is the FortiGate on the endpoint side.
Internet Options Add SSL-VPN gateway URL to Trusted Sites
Go to the Security tab in Internet Options and choose Trusted sites then click the button Sites. Insert the SSL-VPN gateway URL into Add this website to the zone and click Add.
Note: The default Fortinet certificate for SSL VPN was used here, but using a validated certificate won’t make a difference.
Furthermore, the SSL state must be reset, go to tab Content under Certificates. Click the Clear SSL state button.
Internet Options Clear SSL state
The SSL VPN connection should now be possible with the FortiClient version 6 or later, on Windows Server 2016 or later, also on Windows 10.
Don’t get success yet ?
If you haven’t had any success up to this point, don’t despair now, there is more help available, may the following is the case!
Credentials or SSLVPN configuration is wrong
If you may use an FortiClient 7 on Windows 10 or Windows 11, then create a new local user on the FortiGate and add it to the SSL-VPN group.
create a new local user on the FortiGate
Add the user to the SSLVPN group assigned in the SSL VPN settings.Try to verify the credentails using the web mode, for this in SSL-VPN Portals the Web Mode must my enabled.
Note that the group with the affected user is assigned under SSL-VPN Settings at Authentication/Portal Mapping.
FortiGate SSL-VPN Settings Authentication/Portal Mapping
Try to authenticate the vpn connection with this user.