Amtsgericht is the German word for a "law office", or something like that.
Have a look at this discussion from the TT chat forum: Problem landlords
Requirement: Resident
First get a lawyer.
If you are on a needs-basis, the lawyer can direct you to the local amtsgericht to seek payment. Unless you are fluent in German, bring a good friend who is. You may wait up to a few hours. They issue receipts per specific issues.