AirBnB is part of the problem btw. There are at least 4 fulltime AirBnB flats on my building floor (that I'm aware of). I'm assuming it's AirBnB because there's a very high "fluctuation" of new people in those flats (few days to 2 or 3 weeks). Imagine if all those AirBnBs would be available for longterm renting ;)
Since 2014 you need to have an official permission to use a flat for AirBnb. If you suspect these are illegally rented out, you can report them to the Berlin government here:
It is like saying refugees are part of the problem, because they are taking flats from the market. People who rent Airbnb also have a right to be in Berlin.
And this policy is pushing more flats from normal market into airbnbs.