I am in Raedric's keep. I just picked up a new fluffy pet, and suddendly, my inventory does not work anymore. I press i, or i open it from the menu, the result is the same: the game seems to pause but the inventory simly does not open. if i press I or Escape, it unpauses...

This was not an actual bug. After a black-out, I restarted my computer. The game was running very slowly, the inventory did actually open, just after a minute or so. Another restart of the computer solved the problem.