There was an annoying bug (even though it was a licensed copy from Buka) when killing Robert the Wise; the cube wasn't in his inventory :( Loading a relatively recent save (where the quest hadn't been started yet) helped.
I don't know: the game is licensed, and I have the same bug with the cube; I restarted, but the cube wasn't in it. I started over, but it didn't help. What should I do?
