Yeah, I've been getting the same behavior here (12.6 Monterey, FortiClient VPN 7.0.7.0245)
TBH the solution from Fortigate is ridiculously complicated and not suitable to roll out to end users. The easy solution that worked for me was just setup LetsEncrypt to issue a genuine certificate. If the old ones need to be deleted, this was useful:
Hello everybody, thanks for the info, I tried to follow the guide and all the advice, however the VPN does not work from my Mac. In the end I took a (Windows) Pc where the VPN works. I'll trying stuff until it works later...
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