Hi pjassogc,
first things first:
On Windows it asks me to accept the certificate
This should not happen. That indicates that the FortiGate is supplying a certificate to secure the connection, where the client is not trusting it. So you get the warning about it.
Normally such a warning is to be considered serious as it should never appear. If an attacker somehow gets hold of your connection, supplies an invalid certificate since this is required, the warning would then tell you that and make you vary to continue. If the warning is just accepted anyway, an attacker may have passed the first hurdle.
So see with the FortiGate administrator to supply a valid certificate and trusted certificate chain to avoid the warning. The FortiClient on Linux might then also start working.
If not, then debug on the FortiGate may tell more:
diag debug console timestamp enable
diag debug app fnbamd -1
diag debug app sslvpn -1
diag debug enable
You might also test other FortiClient versions, you did not specify any version here. I'm running latest 7.2.3 on a Linux Mint, latest version with SSLVPN and no certificate warnings and it works just fine.
For technical understanding of certificates, we'd written an article about this:
https://community.fortinet.com/t5/FortiGate/Technical-Tip-TLS-and-the-use-of-Digital-Certificates/ta...
Best regards,
Markus
