Sewage is leaking - water is running down the basement wall

Unfortunately, the invoice says nothing about that..

Then the other way round ...

we moved into our new house 4 months ago. The plumber completely installed water / sewage pipes for us.

What exactly does moving into a new house mean? Did you buy a pre-owned property and have the plumbing trade renewed, have a house built via an architect in individual trade contracts, or buy a new building via a general contractor/developer? Was there a formal acceptance? Basically, all forms have in common that the craftsman owes you 2 years warranty; if you built via a general contractor, the warranty period depends on the contract and what is agreed therein. Under a Construction Code contract, the period is 5 years. So the question arises, who must be the addressee of your complaint about defects. Regardless, the plumbing company has the right to repair one or two times; but it can also decide to redo the work immediately. I do not know whether I should deduce from your text that the plumbing company has already made one or several attempts to fix the damage. Usually, a request for defect rectification is first made verbally or via e-mail; if this does not work, a complaint about defects must be issued and delivered by registered mail with return receipt and deadline setting. As your description reads so far and the craftsman is still not willing to fix the damage, you will have to consult a lawyer. The question of the next step must be clarified. But I am not allowed to provide legal advice, as this is exclusively reserved for advisory professions in Germany! Rhineland greetings

Thanks in advance for the answers.

So, briefly for a better description:
We have newly built a single-family house. We awarded the trades ourselves. No general contractor / developer, etc.

The water installation was completely done by the plumber. We connected the washbasins, etc. ourselves, but nothing on the pipes.

He has looked at it several times now. The first assumption was that the leak comes from the wastewater pipe of a hand washbasin. He filled this pipe with a compound and tried to seal it.
It didn't help.

All communication was oral.

I think I will now send the notice of defects in writing and set a deadline for the rectification.