I hope this review makes its way to the FortiClient team and they can fix the issue for all users. They also don’t provide any way to contact them or provide feedback. I understand that this sounds like a forum issue or a support ticket, but the FortiClient team is separate from the FortiGate support team and does not provide VPN support for users of the free app.
After troubleshooting the issue with FortiGate support, it was identified that the 3-way handshake does not complete with a “Reset” flag (value of 1) sent from the app back to the FortiGate device and the connection is dropped. The authentication (including 2FA) works and then the device presents an “internal error”. I have a FortiGate FTG-201F and cannot connect to the SSL-VPN from an iOS/ iPadOS device using this app.