Almost all available accommodations are available only for people exchanging apartments and not starting new contracts. It is hardly ever possible to find an apartment or place in a shared apartment which is not short-term and allows Anmelding. But if you have money to pay an estate agent (e.g. like my PhD friend) it appears that there are plenty of flats available, the problem is that the landlords don’t want to deal with looking for tenants so they never post their offers anywhere and real estate agents have a huge database which you can get access to only if you pay them money.