Have you tried the Wan port repair tool in below link
Technical Tip: The VPN server may be unreachable (... - Fortinet Community
I have the same error, I tried the WAN port repair tool and it did not fix it.
Hi, in general for VPN failures, it can be due to various number of reasons, such as:
1. Whether this is a IPsec or SSL VPN connection
2. What kind of authentication is involved, and if 2FA is used.
3. Is server certificate used?
4. Port used to to connect to your VPN gateway.
This would require some troubleshooting and reviewing your configurations on the FortiGate side as well as taking various debugs. Since this requires looking at your settings, it is better to open a support ticket on it.
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