A public liability insurance is very advisable. It is called Haftpflichtversicherung here. I don't know if a normal Haftpflichtversicherung covers commercial activities, too.

Organisers often write "Versicherung ist Sache der Teilnehmer" (insurance is concern of the participants) on entry forms. This isn't enforcable in court - I think this is either widespread because it's an old habit or because it protects the organiser from damages not covered by the public liability insurance...