NetworkManager-fortivpn - adding 'trusted-cert' to config file
Solution 1:
In the Network Manager (I'm on Ubuntu 18) and configuring an openfortivpn connection, the Trusted Certificate (digest) field is reached by the Advanced button in the "VPN (fortisslvpn)" thumbnail.
Solution 2:
Ok so good news, this is solved.
All I had to do was restart the neworkmanager service (I actually rebooted my machine at some point, however it has been said that one can run sudo service network-manager restart to restart it).
Now my FortiVPN is connecting and I can access the server :)