Well, I found the cause of the port-forwarding issue: one of the routing hops on my LAN had accidentally been switched to firewall mode, so it was silently dropping the reply packets from the server to the Internet. The net result was that a lot of SYN packets were showing up; the server would ACK, but the ACK would be dropped, and then subsequent SYNs would flag as invalid because the local network stack believed that the next packet should have been a SYN/ACK, not a repeated SYN. So that explains the symptoms noted in packet captures.

The net result is that port forwarding works once again - keycount.piqsoftware.com is back live and pointing to the dummy placeholder page that got put up on my spare Windows server box after Milton blew up.

So tonight I think I'll put Milton up "live" and move the placeholder over (assuming I don't crash from this marathon I'm on, during which I've slept about 2 hours in the last 2 days). All I really need to do to get it finished is install PHP5 and MySQL, and that can be done with minimal disruption to the service of a static placeholder page.