Seems I recall last time this happened, though we had a theory, we never found exactly what was causing it.
Our theory was that, mid-game, some common variable was being set as global, instead of remaining local.
That is, we're pretty sure it was someone elses broken code.
Imagine a for loop in lua.
function()
for i, k in pairs (blah)
end
end
In that loop, i should always be local to that function.
Imagine someone in another addon setting i = <global var>.
It would break stuff.
Perhaps Megiddo/Stick/MrP can provide some ideas to help you help us find the problem.
Unfortunately, I don't remember our path for troubleshooting from few years ago (other than some eventually fixing it by removing a ton of plugins, but never sure which one)
EDIT:
Also, make sure you have our latest SVN code.
Garry broke some file I/O lately, we've hacked around it and made our own.
Perhaps the reading of a file goes awry mid-game when someone joins.