If you have detailed diagnostics from the Jabber Mac client, this would provide some more context to why it's displaying those errors. (Help > Detailed Logging enabled) (Help > Report a problem)
Another thing to check for would be DNS resolution of the configured servers when the Mac is VPN'd in. If Jabber cannot resolve the DNS name, it will not know where to connect to.
If the diagnostics are pointing towards a connectivity problem, but the firewall says it's wide open, then taking a packet capture on the Mac where Jabber is trying to register may illustrate what's going on at the network layer.