Deponia bug

I started a point and click game called Deponia, and I'm quite frustrated... For some reasons, a npc mysteriously disappeared of my game and I just can't go further into the game, I'm stuck! The npc still exist because I can still interact with him, but he's invisible and I can't do anything with him ! I'm supposed to burn something and doing this should unlock a new dialog and should allow me to continue the story, but nothing happens because he's invisible, even though I know he is still there in front of me (very confusing, I know I'm confused too).

I'm disappointed because I'll need to restart the game, even though I finished 1/3 of the game.