From when is the fixed price no longer changeable

Why don't you just write that right away? Then it is more likely in my opinion that the FP still applies. But if you do receive a change to the FP, can you prove the delay caused by the contractor? In today's situation, it can always happen that you may expect a price adjustment. At least recently, some such cases have occurred here. But hopefully you have good arguments (evidence) that the contractor is responsible for the price increases themselves.

We can prove the delays seamlessly through written email correspondence.

In the last addendum, for minor changes, we also received a current contract amount of xxx€ which corresponds to the price of the fixed price guarantee with an issue date of 03/18/2022.

The delays can be fully proven by written email correspondence. We have recorded and saved everything in this regard.

Often the client does not see his own fault.

There were also some planning problems because the windows we wanted could not be implemented, but we were only informed about this 3 months after the planning.

That the year 2021 has meanwhile passed needs no proof. Did you reprimand the delays, establish default in this regard, demand the end of the delays, threaten penalties (if agreed at all) and the like?

“Unfortunately” we only verbally reprimanded the default on the phone and initially trusted the construction company that the mills were grinding within the company. We only have email correspondence in which the construction company admits failures due to internal problems.

What was the basis for this extension of time, and what were the change orders about anyway? (this all sounds very much like "planning ex post") – don’t let them pull everything out of your nose!