For anybody who runs into a similar problem:
I identified the problem to be the Polylang plugin which happens to be the currently only reasonably working multi-language plugin.
The problem is that Polylang filters all created topics and replies based on the currently set user language.
By default it does not assign a language ID at all, therefore new topics are not listed. Assigning the correct language from the topics administration fixes that problem but on a very busy website this is not a solution.
