what are some common causes of this runtime error so that i can try to narrow things down?

"Connection died" isn't a runtime error at all. It's the message Dream Seeker displays to people when they are forcibly removed from the game (e.g. by booting). If it's happening when you don't want it to, it's probably because you're deleting the player's mob or client by accident.

If your game has character handling and the problem occurs when logging in, it might be because you're deleting the player's mob before logging them in to a different mob (as opposed to doing it afterwards).

If you want more information than that, a bit of context for the problem would help. =)