Hello firecat,
thank you for the time you have spent on getting those information, this is very much appreciated. BIG thanks!
I have redone the tax interview and now it says "Withholding rate 0". The tax interview abstracts the form and takes you through it step by step. You only get to see the actual form (filled out) after answering every question of the interview. Looks like the important part was "6". I thought that I would have to check the box that says that I don't have a TIN or an equivalent of it which took me to the wrong follow up page of the interview. After stating that I have one, I was able to enter my German "Steuernummer" (equivalent to SSN in the USA). There was a ckeckbox to mark the number as a SSN but it could only be marked as checked if I had entered it into the US citizen related field but my German number was accepted in the foreign number field and I was finally able submit the form. Puh!
I read that in Germany there is a rule that if you only earn up to 410€ per month in addition to your main job it is sufficient to note the amount on your annual tax declaration form. Only when you exceed the amount, you need to found a "Kleingewerbe" or a company.
Since I don't expect to earn that much that should be all I need to do in order to comply to the law.