@Sten1hopefully you got through this ok. Due to the harsh penalties in Germany you should have every guest sign an internet waiver on arrival. To fix the above issue, unfortunately you need to take a lawyer to prove you were not there and that it was another person. You need to provide the Airbnb booking details to prove this. As the Airbnb guest has broken the law, you can also get Airbnb to act on your behalf, not that they will get the money for you, but it can lead to Airbnb suspending the guests airbnb account. I have been through a few of these hoops here.