Same thing—backpack and menu randomly disappear. First time it happened, I couldn't save the game, had to quit via task manager, and had to start from scratch next time I played. Second time, it got me stuck in the cave because I couldn't access the magazine (I had a hunch I should do that) after I'd picked it up, and I couldn't save or quit the game either. Again, I had to use the task manager to quit. So that's a really nasty bug.
But I played the game through, at last, and I really enjoyed it.