BÜ wants a declaration of assignment. How to avoid it?

It can't work like this somehow.
You should trust a partner. Something must be right for you to give him the benefit of the doubt in the first place.

What would you want to do if the entrepreneur does not want to keep the contract?

Could it be that you have never had trust and are therefore quite uncomfortable? Since you don't give us an explanation either (which you are not obliged to), you should reflect on yourselves.

These are questions one should clarify for oneself. You owe us no accountability, but you do owe it to yourself.

What is the name of your contracting partner?

You can answer this question for us.

My question was not who is right or wrong, but whether there are ways to save the situation and spare us the assignment declaration / guarantee.

I would say: that is your learning fee, the consequence of it.

The same applies to you: if necessary, create a separate thread ABOUT the problems with the GU/BU mentioning the GU collectively in the moderated forum, rather than discussing problems bit by bit and having them repeatedly occur negatively, even though they have been neutralized.

I assume that the late payment would not have been a problem either, if it hadn't been for the problems with the architect and the soil report afterwards. The demand for the assignment declaration probably arose from the fact that the contractor suspects that you are the type of client who withholds payments over every little thing.

I assume that the late payment would not have been a problem either, if it hadn't been for the problems with the architect and the soil survey afterwards. The demand for the assignment declaration probably arose from the construction contractor suspecting that you are the type of client who withholds payments over every little thing.

Really speechless about this comment and the conclusion drawn about people one does not even know. o_O:D I still believe that many here get the tone wrong and assume the role of a judge, especially when they cannot contribute anything substantive to the question. Not meant to offend or take personally, just an observation.

No one here is in a position to make any judgments or to speculate. That doesn't help anyone.
My advice to you again: get paid expertise, represent your own interests factually and seeking compromises (or represent them uncompromisingly according to the experts' advice), and otherwise strive for a very friendly tone and interaction with the company.