Replace sink with pre-wall element

One would think that the previous tenants have to restore the original condition when moving out. But the issue is probably already over and done with :-(

Are you a tenant?
If yes, why are you taking care of it? That is the landlord's responsibility.

The landlord accepted the apartment from the previous tenant in this condition. And I also accepted it this way at the handover... so I am hesitant to complain about it now. But on the other hand: Who gets under the sink during an apartment handover to check the fixtures?

My plan is to first have the plumber take a look; he’s coming next week, and if he can’t do anything, to inform the landlord. Still: If anyone has any tips on how to solve this technically, just let me know, maybe I can then pass something helpful on to the plumber.

Hi,
either way, not your problem.
If you are a tenant, a sink is the landlord's responsibility; a properly installed sink is part of the apartment!
If you are the landlord, the tenant has to restore the apartment accordingly, so withhold the deposit.
Regards, Olli

You rented the apartment without a sink? Then that is actually your problem, as the rule here is rented as seen.

In my apartment, the previous tenants removed the washbasin that was mounted on a pre-wall element.

Who lies under the washbasin during an apartment handover to check the fittings?

So what now? Did you take over the apartment with or without the washbasin?

Apart from that, regarding the fittings, it doesn't really matter whether the washbasin was there or not. That is a defect that the landlord has to take care of (the defective fitting).