Termination of basement construction contract - experiences?

I have now asked the two representatives of Kellerbauer and house manufacturer to simply have a phone call with each other. I hope that the problems simply arise from the fact that I, as a layperson, always have to pass on information and requirements (Chinese whispers principle) and that it simply cannot work that way. Both are reputable companies, so there must be a solution?

It's not that simple. They don't even have a contract with each other. You have the duty to coordinate if you haven't commissioned an auxiliary agent. You are the client. If coordination turns into a game of telephone, that's your failure.

Correct. You can invite to a joint conversation. But not say "I have a problem, now please solve it by talking to each other on the phone!"

you won't believe it, I know that.

You have just bought individual trades together and since there is no overall responsible professional, you as a layman are responsible for the whole thing. Every contractor first looks at their own trade. It remains to be seen whether the two trades will agree.

Somehow the sentence sounds to me like: "Wash me, but don't get me wet!" Why shouldn't the basement be officially included in the thermal envelope? If you insulate that part, then from my point of view you have to basically temper the entire basement anyway. How is the thermal separation between the living area and basement corridor carried out in the stair area?

For me, there are only 2 options. Either an uninsulated utility basement outside the thermal envelope or a living basement inside the thermal envelope. You somehow want a mixture of both and in my opinion this can end up like a dripstone cave.

It's not that simple. They don't even have a contract with each other. You have the coordination duty if you haven't hired an agent to fulfill the service. You are the client. If coordination turns into a game of telephone, that is your failure.