I have the same bug.
What happens in more detail is this:
1- I ask the question to a merchant
2- The camera shifts to my character
3- The game stops responding to any mouse or keyboard input. It does keep running however, since the 3D character keeps moving and the music playing. No keys, including ESC, nor any mouse input do anything anymore.
Reloading the save just before this happens makes the bug reoccur. However, going to another merchant can solve it. It seems this is the case because the other merchant immediately gives the hints, instead of the camera dialog (me asking, him answering). I have never had the bug when the merchant immediately answers.
This bug happens very frequently for me, about every 3 merchants I would guess and is quite a showstopper.