Here is some additional background information for the developers to fix the problem in general: The backend does not allow to add a place only with a sub-category. However, I could add places with the first level category, add a sub-category afterwards and delete the first level category then. Although the place was then left only with a sub-category, no problem occured!
The problem only raised when I had to add places via the frontend due to some problems with locating the places on the backend map. The frontend allows adding a sub-category without adding a first level category first. Doing this caused the problem even for places that I had added via the backend first.