If your journey is of a private nature, then, in the case of flight disputes, you have the right to contact the ‘Schlichtungsstelle für den öffentlichen Personenverkehr e.V.’ (SÖP), Germany’s independent arbitration board for public passenger transport that oversees all carriers.
You may do so if you have concerns relating to
The postal address is:
söp_Schlichtungsstelle für den öffentlichen Personenverkehr e.V.
Fasanenstraße 81
10623 Berlin
The European Commission also provides a platform for Online Dispute Resolution (ODR):