Interior door noticeably warped after installation

Hello everyone,
I need your advice on a topic that is currently very annoying me.

We moved into our new house at the end of last year (new development via a developer). As requested, the interior doors were not installed at the time of handover but were to be installed by us afterward (after completing our own work).

Accordingly, we installed the interior doors ourselves at the beginning of this year, as the last trade (the rooms were painted, the floor laid). I also confirmed the apparently proper installation of all doors with the installer.
A few months later, we have now noticed that one interior door (from the living room to the basement) has warped significantly. The door fits flush in the lock but visibly sticks out at the top and bottom (like a U – see photo).
We reported this via the developer’s defect portal, and the matter was passed on to the door company. Without even looking at the door or at least consulting with us, the defect was coldly rejected.

Reason:
The door was obviously in order when installed, and a factory defect could be ruled out because only one door was affected. They assume improper handling by the customer (wrong storage of the door, failure to comply with climatic conditions, etc.) and therefore are not responsible. A door warping of up to 4 mm is also permissible.

Only after our renewed request did someone from the company finally come out to at least take a look on site. The installer found a warping of 9 mm as proven. Nevertheless, the defect is still being rejected with the above justification.

Our arguments that it is completely unrealistic to permanently remove an interior door in a new building (where all work is now completed), especially the basement door with a small child in the household, or whatever else one might do with it, were completely ignored.
We were simply offered a new interior door at our own expense.

The developer, of course, immediately sided with this statement and also rejected the defect.

Now I am a bit at a loss. Here it is a case of one statement against another, whereas I believe that there is indeed a verifiable defect and that we are definitely blameless, especially since the other 13 interior doors are not warped in this way.

Can the company still so easily dodge responsibility, and are we stuck with the damage? Every other trade has examined the reported defects and also repaired them accordingly.

I would appreciate any experience you might want to share.

Many thanks,

Christian

A few months later, we have now noticed that an interior door (from the living room to the basement) has warped significantly.

Basement heated or a "cold" basement? A cold one combined with a simple quality of the door can cause this. Otherwise, it is not normal. What amount is involved with the new door leaf?

Yeah yeah, the industry is probably doing (still) too well. This arrogance is outrageous.

If the door is supposed to be "inferior" for the installation location, the BT is not allowed to install it. At least not without having a liability waiver signed beforehand.

And since it's about the basement staircase door, I don't think that's the problem.
maybe you have temp. and RH ready from both sides and can argue.
Or threaten (hint) with bad "press" on the internet.

What amount is involved with the new door leaf?

That could of course be the "problem," that the nerve war is in no way proportional to the value.

So the basement is also heated and there was not a big temperature difference. Of course, I can’t prove that either. Sure, a new door leaf doesn’t cost the world. About 120 euros. But honestly, it’s about the principle for me. We have nothing to be blamed for and still have to bear the damage. It just can’t be that simple. But a lawyer / expert can’t be the solution either. Although I would argue, if the lawyer writes a letter, there will be a new door leaf given out of “Kulanz”.

Yes, I can imagine that too. As I said, if it's not too dumb for you, feel free to cause a bit of trouble. You have my full moral support ;)

But honestly, for me it's about the principle

Lawyers live from personal principles

But a lawyer / expert can’t be the solution either. Although I would claim, if the lawyer writes a letter, a new door leaf will be provided out of "goodwill".

A lawyer’s letter from my copyright lawyer cost €75 + VAT back then 12 years ago. Calculate that for yourself. It doesn’t stop at one letter. Just try it, but please not through that online thing, rather an official notice of defects with a deadline (you can google what that is and how it works. It requires a form because it is legally stipulated). Argument: how could only one door warp during storage if the defect did not appear in the other 12 doors despite the same storage.