Banks are not obliged to let you open a business account with them, unlike private accounts. What reason have they given for refusing you? Have you tried at both traditional and online banks?
Have you asked to see your credit rating from SCHUFA? You can request this free of charge here:
Here is an example of an expert answering a question about this on a site for people setting up a business:
The site offers some useful information in English about starting a business: