The situation in Berlin before the policy change was that the city government had made it very hard to build apartments at all. Each time a project it developed, the developer is blackmailed into supporting various unrelated policies, being it parking space related, maximum stories, etc., and sometimes capriciously denied permission to build. This is how my green-leaning, government-trusting architect friends describe the situation.
This happened all the while time Berlin changed from being viewed as a forgotten backyard in Germany with few companies and high unemployment to the capital city of the dominant and richest country in Europe. Especially the great financial crisis accelerated this change of perception.
Consequently, many people moved to Berlin. For many years 100k people per year moved into a city of 4m. Stated in terms of economics, supply of apartments was very steady, and demand jumped up.
The sound response of a market is to signal that through price hikes. That should encourage investment in residential building. When this is being suppressed nothing can even get better. Only longtime renters are favored. People with already cheap and large apartments benefit. Losers are those young couples expecting a baby.
A solution that could potentially work would be to accept temporary pains from high prices, remove regulatory obstacles from new construction and let the industry find a new balance. Maybe accelerate the process by governmental subsidies, if you like that sort of policy.
But the current policy can't work in theory, it doesn't work in practice. And it only benefits old established interests who are privileged already.