I have received a copyright infringement alert, what should I answer?

Yeah. Hetzner is the hosting company. They are the owner of the IP range and thus get the letters. They forward it to their customers, in this case OP. And the letter seems to be from one of those shady companies that scan the torrent swarms for Intellectual Property of their customers and then write letters to the abuse contacts of the IP addresses of the offenders. I don't know where OP lives, but Hetzner is big in Germany, so it's probably german law we're talking about. And we're not very liberal with copyright infringement, should that escalate to that point.

Thanks.