I'm not using an Airport. The firewall on the network is running DD-WRT and no settings have changed since it was working with ML Server. I did test the problem with an Airport Express, though, and was able to reproduce it with that equipment as well.
My server does not have a public address. VPN traffic is forwarded from the firewall. I do see activity when the connection attempt begins, but it never succeeds.
Oct 23 00:31:47 servername racoon[14219]: accepted connection on vpn control socket.
Oct 23 00:32:02 servername racoon[14219]: Connecting.
Oct 23 00:32:02 servername racoon[14219]: IPSec Phase 1 started (Initiated by peer).
Oct 23 00:32:02 servername racoon[14219]: IKE Packet: receive success. (Responder, Main-Mode message 1).
Oct 23 00:32:02 servername racoon[14219]: >>>>> phase change status = Phase 1 started by us
Oct 23 00:32:02 servername racoon[14219]: IKE Packet: transmit success. (Responder, Main-Mode message 2).
Oct 23 00:32:03 servername racoon[14219]: IKE Packet: receive success. (Responder, Main-Mode message 3).
Oct 23 00:32:03 servername racoon[14219]: IKE Packet: transmit success. (Responder, Main-Mode message 4).
Oct 23 00:32:03 servername racoon[14219]: Connecting.
Oct 23 00:32:06 servername racoon[14219]: IKE Packet: transmit success. (Phase 1 Retransmit).
Oct 23 00:32:24 --- last message repeated 3 times ---
Oct 23 00:32:24 servername com.apple.launchd.peruser.502[4494] (com.apple.KerberosHelper.LKDCHelper[47084]): Exited with code: 1
Oct 23 00:32:41 servername racoon[14219]: IKE Packet: transmit success. (Phase 1 Retransmit).
Yep, that's all of it. No real indication of failure even.