Hi @marc.polatschek
Thanks for reaching out to the Zoom Developer Forum, I am happy to help here!
It looks like you are using the wrong set of credentials.
You do not need to pass the code or redirect_uri to generate access_Tokens with the Server to Server OAuth app.
Here is a topic that I wrote on how to use the Server to Server OAuth app type with Postman.
This might help you understand the issue you are seeing
Best,
Elisa