Re: FortiGate to StrongSWAN: "Failed to find IPSec Common"
It would help to id if you have diag vpn tunnel output and proxy proxy-id between the two private-Networks
Also , if ( and it looks like a yes ) this is a route base, you can dump on the interface directly in FortiOS to look for traffic entering and leaving the interfaces. I would also triple check SNAT is not been mistakenly added to the mix and if the two private-Networks are not be masked behind a nat'd address
Good to see others using StrongSwan ;)