Ok, after a couple of weeks I wanted to play again and the bug was still there. I reloaded and now it works, so here is my tipp for anyone else who meets this bug:
IProblem: I FIRST unleashed Henrik and his "family" (sneaked in the house) and fought after that against the bandits. I think that was the problem, but I'm not sure. After you kill the bandits, you get a quest "unleash them". I did this before I killed the bandits and I think the game couldn't handle this. So: Kill
Solution: Kill all bandits, THEN unsleashunleash the others. :)
If it's already to late for you, I think reloading an old savegame will be inevitable.