Pipe burst or what could it be? Who has ideas/tips?

Dear experts,

I registered here with my question and hope for an answer.

We moved into a new apartment 3 weeks ago. We knew the previous tenants very well. For a few days now, we have noticed a continuous dark stripe on a wall downstairs in the new apartment, which runs through several rooms and was not present with the previous tenants or when we moved in.

Could it possibly be a pipe leak? In my opinion, only one spot would look like that, but through the entire wall? Unfortunately, I am at a loss.

What can you say about it at first glance?

I look forward to your answers.

Best regards

Horizontal, at this height? - that would, if at all, only fit with heating pipes.

Report to the landlord immediately!

What did you install?

There is water on the floor and it is already rising up the wall. I would be surprised if the damage is not even greater among you. Report to the landlord immediately and possibly also turn off the water until the cause is found.

Looks like a defect in the underfloor heating.