From when is the fixed price no longer changeable

In my mind’s movie right now the film is running: client wants floor-to-ceiling windows (which, simply put, leads to "structural problems" and also poses special challenges for the window manufacturer), but settles for a drawing assistant plan by the general contractor ...

It was only a change of a shower and no intervention in the statics

If the “fault” of the contractor is also evident, then everything is fine. Often the client does not see their own fault. And then a written document with deadlines, clear formulations, etc. is missing ... An email correspondence can be flawless for all that.

Unfortunately, we never set deadlines for such matters since we were always assured of new dates. The building application alone was postponed by more than two months and was submitted incomplete. The change of the shower also took place after the building application was approved and, according to the construction company, does not pose a problem in this regard.

Unfortunately, we never set deadlines for such things because we were always assured of new dates.

Yes, I believe that. Such things should also be done by registered mail, that is, if you notice that something is not going right.

The building application alone was postponed by more than two months

However, we do not know why something was postponed from where/when. Was there a deadline for submitting the building application or how do the mentioned 2 months come about?

and it was also submitted incomplete.

You sign the building application. What was missing? Normally, something is submitted later…

The change to the shower also happened afterwards

… but that has to be redrawn in the execution planning. I don’t want to defend the company you chose for certain reasons now. But you have to reflect a little more precisely on yourself and your actions before looking for blame. There are also too many factors that play a role here. Maybe you have to wait your turn because your house is not lucrative for the construction company.