Wolfgang has always been very easily reachable by email, although he was not in Berlin. He answered very quickly as well as his flatmates who were very helpful and hospitable.
The flat is very nice and 10 minutes far from the metro station.
Only one drawback: the matress of the sofa bed is average even if it was really okay for us but if you have pain in the back, it may be annoying. But in this case you can still use the loft bed.
A really good address to visit Berlin!