Claus Kiefer, "Quantum gravity", 3rd ed., page 220/221, says in chapter 7 "Quantization of black holes":
"A theory of quantum gravity should give a definite answer to the question of whether unitarity (with respect to an outside observer) is preserved or not."
I am not able to see the problem here. According to the basic characteristics of a black hole, an outside observer can never observe what is happening inside the event horizon, and for everything happening outside the event horizon there is no unitarity issue.
So is there an error in the cited phrase or am I missing something? What exactly should a theory of quantum gravity provide?