It's when I connect. Idk if you heard about the TNT incident, but my place got rolled back to before the 1.6 update, including some chests. I think there's stuff in there that 1.6 doesn't recognize, cuz it crashes.
I had this problem before in singleplayer when it switched from 1.3 to 1.4. I ended up having to destroy the chests in a 3rd party editing program. I'm thinking a mod can just turn that whole area into dirt and it should take care of the problem.