Is exclusion of warranty possible in the offer for time and material work?

The problem is more about enforcing the law - getting upset like an HB-male is always possible, but if the company doesn't respond, you're left standing there.
Talk to them again about what exactly they want to exclude - a defect in the shower (the thing) itself is clear, but the connection cannot be excluded.

: I think so too, but I am just not sure whether the warranty can be excluded by announcement in the offer according to the law.

A look into the law often facilitates legal determination;

§ 639 Building Code Exclusion of liability
The contractor cannot invoke an agreement that excludes or limits the customer’s rights due to a defect, insofar as he has fraudulently concealed the defect or has given a guarantee for the condition of the work.

This means conversely:
Without fraud and guarantee, (individual) exclusion is possible.
Limit: Standard terms and conditions contract law (small print, model contract).

I consider the exclusion here not objectively reasonable, but legally it is possible. If one does not want this, the consequence would be to change the provider.

Stupid question but is this stress worth it? What was the difference between [Sanitär] and [Internet]?