After some experiment, I think I figured when this error occurs. After setting a balloon on an NPC, if you you later clear the balloon using "clear NPC balloon" command, save the game and exit, and then reload the save and try to enter the map where the NPC balloon was triggered, it gives you an error.
Edit: to be more precise, entering the map where the NPC with the balloon icon on top of it that was cleared by "Clear balloon" command, gives an error.