[Lackie]

What appears to be happening is that your traffic is coming in firewall a into the internal network and then over to the internal interface of firewall b. Firewall b gets the packet but knows that from that source it should be encrypted, not in cleartext as it is receiving it, hence the error message that you are getting. Everything is happening in reverse when firewall b handles the initial packet.

Not sure if you are using a static or dynamic work assignment for your cluster. Changing it to static may help, but I'm not in a position to test this. Also check the 3rd party configuration on the cluster object, is Sticky connections enabled?