Unfortunately, Microsoft isn't disclosing exactly what the problems are. When asked by a user why the update was pulled, a Microsoft spokesman said:
Obviously, Microsoft is trying to downplay the situation, and it feels the best way to do that is to give absolutely no information about what is happening other than to say it only affects "a limited number of users". It would seem to be a fairly severe issue, even if it only affects a few users, if the best option is for Microsoft to pull the update entirely.
No word on when the update is back in the Store, but we'll keep an eye out.
source: Microsoft via Neowin