I stayed within Ulrich's flat for ten days. I'm listing below Pros and Cons of the apartment.
PROS: the flat is conveniently situated in a pretty area of Berlin, five minutes of walking distance from the tube and from a major supermarket; plenty of cozy restaurants and pubs in the immediate surroundings. Ulrich was very helpful and quick to answer me when I contacted him. I couldn't hear any noise whatsoever during the night coming from the upper flats.
CONS: the flat is situated in the basement of the building and the windows opens in a closed yard, surrounded by the walls of the adjacent buildings. This means that it is impossible to directly see the sky and to freely and wide open the windows (this would have implied that people passing by in the yard could have had a look inside). At the end, not being able to see the direct light of the sun and to properly ventilate the flat turned out to be rather discomforting.