I'll answer to myself, since TCat doesn't know and rather wants to "move on" now, after I asked for chapter and verse of the law.
The rule in question seems to be the "Berlin Freedom of Assembly Act" (approx 2020) which, interesting enough, further constrains the right of assembly as guaranteed by the Grundgesetz. I stll have my doubts if a federal country can do that. But I'd like to point out that the GG Article 8 only guarantees the right of assembly to all _Germans_.