Hi @floeH,
Would you please share information about your german Office 365 license type, Enterprise E3, Enterprise E5 or the others?
I just created a Office 365 Enterprise E3 free trial, then use this account to sign in Microsoft Flow, works from my side.
If you are using Office 365 Enterprise E3 or E5, then please sign in directly, there is no need to register with that account, you already have the Microsoft Flow license.
What you may need to check is whether the Microsoft Flow license is allowed under Office 365 Admin center for the account you are working with.

Adding some articles for your reference:
Sign up and sign in for Microsoft Flow
Microsoft Flow plan
If you have any further questions, please post back.
Regards
Michael