I'm having the same issue with getting a Fortinet to connect to ASA.
If I protect 1 network then the VPN connects, but if I have multiple, it fails
I got mine working, my issue ended up being a NAT configuration issue on the ASA side. Once I resolved the issue, the tunnel came up and stayed up
Many thanks for that hint. In any circumstance, my ASA was using the same NAT-IP for two hosts. Very strange behaviour.
I'm using object NAT. The (right!) NAT object was used at the NAT section of the host.
But in the object overview i saw, that it was the IP of another NAT object. Creepy! :-)
Make sure to create a NAT entry to NOT NAT any traffic from your "protected Network" to the remote Network. The order of the NAT entries is important also (that was my problem)
Having exactly the same issue with Barracuda NG and ASA 9.7(1)15.
Can't understand, that Severity of this bug is "Enhancement".
I have this problem also with Cisco IOS 15.4 on 800 series with strongswan as the remote end.