[It looks like the update fixed it! Yay!]
While trying to exit the bat cave north of snow town, I accidentally went through the black hole on the left instead of the bright one on the right, and the game immediately closed. Now whenever I try to log on as that character, the game just closes.
I'm using Ubuntu Linux, if that has anything to do with it. But I'm thinking it's more of an in-game glitch...