Confirmed the disconnection issue today with b61. It was a different player this time, but the same symptoms. The player was having intermittent issues with his internet connection, and the connection finally dropped which caused the server to lock up (transfer status showing constant green; no updates to any players). He reported no applications open other than MT and Skype, and felt pretty sure that he was just having trouble with his internet connection. I did have a browser open server-side, but nothing exciting running (just Gmail and the D&D compendium). Upon trying to close MT I got the mysterious empty error windows and had to kill the application manually. Log showed an error closing the UPnP port, nothing else.
If there's anything I can do to help figure this out, please don't hesitate to ask. I'll be happy to run a server whenever it's convenient for you - my schedule is pretty flexible right now. I'll post a server for the next hour or so, under the name DiscoTest (no password). Feel free to hop on if you happen to be around. Skype name is bradymikep.
I had this problem too. I'm not really sure what happened but one minute everything was fine, the next people weren't receiving any updates anymore. I'd move tokens around and stuff but they didn't see any changes, and I don't think I could see theirs either. When I saved and quit to restart I also saw them empty error dialog.