Remove any Phase 1 or Phase 2 configurations that are not in use. If a duplicate instance of the VPN tunnel appears on the IPsec Monitor, reboot your FortiGate unit to try and clear the entry. If you are still unable to connect to the VPN tunnel, run the following diagnostic command in the CLI:
Nov 12, 2019 FortiGate CLI Commands for Troubleshooting - ITAdminGuide.com Mar 30, 2019 ipHouse // Debugging IPSec VPNs in FortiGate The network admin typically doesn't have direct access on the computers on either side of the VPN in order to initiate that traffic. I'll show you a method that can be used to initiate traffic from that network as well. Here are some basic steps to troubleshoot VPNs for FortiGate. In IKE/IPSec, there are two phases to establish the tunnel.
This blog post is a list of common troubleshooting commands I am using on the FortiGate CLI. It is not complete nor very detailled, but provides the basic commands for troubleshooting network related issues that are not resolvable via the GUI. I am not focused on too many memory, process, kernel, etc. details.
And one more IPsec VPN post, again between the Palo Alto Networks firewall and a Fortinet FortiGate, again over IPv6 but this time with IKEv2.It was no problem at all to change from IKEv1 to IKEv2 for this already configured VPN connection between the two different firewall vendors. ipsec vpn between 2 fortinet devices : fortinet
How to Clear IPSec VPN Remote Peer on Cisco IOS
Mar 30, 2019 ipHouse // Debugging IPSec VPNs in FortiGate