the only(!) valid solution to this problem is to replace the expired certificate.
Your VPN server (FortiGate) has that certificate and it expired. This has to be replaced. This is normal for certificates and a security measure.
If the certificate is expired, your client (or any others), do not connect as they refuse the connection and that should be expected.
A very temporary solution to this, if you really need to connect a single time to exchange the certificate, is to change your clients system time to before the date that it expired to.
If not sure where to read it - connect via browser to the same FQDN. You will also receive a warning. Bypass the certificate warning as much as possible and see the date of expiry. Change your MAC OS system time to before that date. Then you should be able to connect.
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