Damages caused by screed work. Who is liable? How to proceed?

By the way, a DIN is not a regulation but a recommendation. So you don't need to bring up such things.

If the DIN is part of the contract. Then it must be complied with.

where are the pictures finally?

This is not about any DIN or photos, but about who caused the damage. That means how something was demonstrably documented in writing by the planner/executor schriftlich. As a rule, a client is involved in the tendering or awarding of the contract; nothing is known about this.

Thank you very much for the tips. The screed layer has fixed the damages. I did not want to upload pictures because I do not want anyone to potentially identify the property. I have often mentioned the construction forum...