Summary of problem; Clients get a timeout error, I may or may not get a disconnection message from some port above 50,000. The router, firewall, and Windows Firewall are set wide-open. I have tried everything I can think of and remain perplexed.
I'm running a 1.5.2 server for a couple friends, everything has been working for months just fine. However, this evening one of them lost connection and couldn't reconnect. Then the other re-logged to see if they had the same problem, and they did. Now neither of them can connect, but can connect to other servers just fine.
Sometimes, when they try to connect I will get a disconnect message with their IP and some huge port number, most of them above 60,000, some between 50k and 60k...but most of the time I don't even get that.
As I said in the summary; Windows Firewall is off, my software firewall is off, and my IP is set as the "DMZ" on our Linksys router.
I've also tried every combination of DMZ and port forwarding. I've tried other ports in drastically different ranges. I've tried rebooting and resetting everything (server, router, modem). I've tried running a completely vanilla server.
When I run an offsite port-scan the status of 25565 is not consistent. Twice it was open, every other time it has been closed or blocked.
I am at wit's end. The only thing I can fathom at this point is that my ISP is selectively blocking Minecraft packets to my IP, but I still think that's unlikely. Please help me find a plausible explanation, if not a solution.