Construction company halts work despite overpayment

The intermediary is a sole proprietorship, the executing construction company is a GmbH with one person.

Why do you need an intermediary there, and what is his legal position in your contractual relationship? A GmbH has its own legal personality, regardless of the number of shareholders.

Instead of clarification, it feels like new inconsistencies keep coming to light :(

Bankruptcy is not an option to be chosen freely - there are criteria for it, and they are either met or not. He would thus have to "prepare" the fact of bankruptcy, but that would be an insolvency law offense.

But this is how the OP expressed herself. The quote "prepare" comes from her, not from Katja.

Isn't it the company Wxx Massivhaus e.K.? That exact company has its construction service description, reference houses, etc. So the online presence.

Isn't this a GÜ... of the GÜ with whom you have the contract, or?????

Ah... ok... I understand... Mr. xxx is an intermediary. So this probably results in a completely unsuitable situation here.

"The general contractor takes on the entire construction management. Overall coordination, procurement, construction supervision, and warranty from a single source guarantee high execution quality with guaranteed construction time. Due to many years of collaboration, proven and competent partners are available for all trades." Source Wxx Solid House

But this is how the OP expressed herself. The quote "prepare" comes from her, not from Katja.

I had quoted Katja's question - not claimed that the original statement came from her.

Ah... ok... I understand...

You are ahead of me - I don't understand anything: Mr. XXX is the general contractor, Mr. XYZ is the broker - but why do you need him here, and which of the gentlemen is the contracting party of the OP? (the one with the new company is the general contractor, not the broker).

The managing director of the contracted construction company said that due to the structural engineering, massive walls instead of drywall now had to be built in the attic. He initially estimated the additional costs at just under €1000, two days later it was €3,802 because he had forgotten a beam in the first calculation. In addition, there was a change in the attic space deemed necessary by him, with approximately €1400 in additional costs. I then informed him that we were not willing to bear these additional costs, as the changes were not requests from the clients and the structural engineering had been calculated by an architect commissioned by him.

Have you even read the construction contract?
It doesn't seem so.
To quote from your construction service description:
"Additional costs that become apparent only after the structural analysis and execution planning due to structural or energetic reasons are not included in the fixed price."

This clause can be found in almost any construction service description. The general contractor usually calculates with a certain amount of reinforcing steel or concrete. If that is not sufficient, the client has to pay the rest. How could it be any different? The general contractor is normally not a clairvoyant and does not know to the kilogram how much reinforcing steel needs to be installed without structural calculation.

Honestly, the general contractor really seems to be an idiot, but if you make demands that are clearly not covered by the construction service description, then in my view you are already partially to blame if the general contractor digs in his heels.