DENIC (who controls .de) has its own (very... German) dispute system: https://www.denic.de/en/service/dispute that they would rather you use. It requires mailing a form to request information, then mailing a form to send a dispute (don't be fooled by the web forms or PDFs, those are just tools to generate the documents you need to print, sign, and send).

Their FAQ lists why you usually don't want to go for UDNDR when you can help it, though: https://www.denic.de/en/faqs/all-faq#code-106

ICANN's procedures are all nice and dandy if all three parties involved are in the USA, but when it comes to international disputes (in this case the Dutch company registering the domains and the German business being impersonated), things can get pretty complex and expensive real fast.