I have posted this question about a month ago. As nobody said anything, I offered a 50 points bounty yesterday. So I got two answers, none correct.
To my surprise, someone returned my points, cancelling the bounty, and probably the same person also downvoted the question without further explanation (this strengthens my point of view that at least the first downvote should be forced to leave a comment, the remaining downvotes should be forced to at least vote in an existing comment).
Why was the bounty returned?