Getting this too on MacOS 12.6 (Monterey), FortiClient VPN 7.0.7. The FortiGate is a 60-E running firmware 7.0.6. No problems connecting to the same server using VPN Client 6.0.1 on Windows 10.
The main thing that's throwing me off is the "Do not warn invalid certificate" option basically doesn't work for newer Macs. So, the certificate must be valid. I got a LetEncrypt cert, installed that, used a hostname that matched the cert, and now it can connect fine.
One thing to watch out for with the cert is it needs to include the chain. For LetEncrypt/CertBot, this is the 'fullchain.pem' file.
To troubleshoot this yourself if you have this error, try eliminate the client as the issue by accessing the web portal through a web browser via xxx.xxx.xxx.xxx:yyy/ where x is your IP and y is your port. Updating FortiClient to the newest version resolved the issue.
Yes, there seems to be different behavior after upgrading FortiGate VPN client from 7.0.7 to 7.0.8. I get a one-time warning about the certificate, and after that, can connect fine without warning. Oddly, the "Do not Warn Invalid Server Certificate" checkbox always seems to remain unchecked.
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