A quest seems to be missing or was misplaced in the data. Krusty has a red and blue exclamation point above his head (as opposed to the standard yellow coloring), and whenever I tap on him to begin the quest, the game will crash. All other characters can be put on quests except for Krusty. If it helps, this problem began about a week ago when I downloaded the Booberella character and began her series of quests.
It isn't a crippling problem with the game; I still enjoy playing it daily! However, I just wanted to bring it to attention in the case other players are experiencing the same problem!