Actually, now that I think about it, here's what happened:
One player got disconnected due to his router issues. I can confirm this because he was simultaneously disconnected from Skype. Shortly after that the other players told me (via Skype) that they weren't getting any updates either. I noticed the Send/Receive panel was running constantly green. Maybe it was blasting packets at the disconnected player, failing to recognize he was gone?
I was able to save the campaign with no issues, but when attempting to close MT a bunch of errors were thrown (no description - just an empty error box). I clicked through about 30-40 but they kept coming. Eventually I had to kill the application. Restarted MT server and everyone reconnected fine, no issues for the rest of the session.
We had the same issue last night. Mac OS X Snow Leopard. One player got disconnected, everyone stopped getting updates. Empty error boxes when attempting to close MT. With a restart, everything worked fine.
log.txt is very uninformative, alas:
2009-10-26 18:20:11,691 [util.ImageManager$BackgroundImageLoader:293] ERROR - BackgroundImageLoader.run(Izzy,279bde201d442a4e01af487e9fc4493e): not resolved; IOException
java.io.IOException: Could not load image: null
2009-10-26 20:49:04,772 [client.MapTool:224] ERROR - Server has disconnected.
I'm using a Linksys router; my firmware is up to date and there's nothing relevant in the router logs. I also haven't changed anything in the router since my last session. The only changed variable that I know about is the MapTool version... hm, no, haven't upgraded anything on the Mac either.