It was probably locked for the upcoming DLC.
If that were the case, I think it would have been locked from the start, not suddenly locked in a patch. Besides, from what I know of the one currently announced DLC, my impression is that it's going to use mostly new locations, not original locations that were inaccessable.
So I'd say that most likely, they just made a mistake in the patch, and it will probably be fixed in the next patch... hopefully, anyway. Because if a patch that's suppsed to fix bugs introduces new ones that weren't in the game before (And I'd say a location that is supposed to be accessable but isn't due to it being "inaccessable" or using a lock that requires a key but for which no key actually exists, it is a bug. Sure, it's an issue with game design rather than a mistake in programming, but it's still a little screwup on the developer's part that makes something in the game not as it was meant to be.), then it's already a problem, and such issues should be fixed as soon as a new patch becomes available.