Cancellation of the installation appointment at Allkauf Haus - costs or compensation?

So I can't say anything about the question of guilt, since I can't judge what/how it was communicated. But as has already been hinted here, also keep in mind that you will still have a lot to do together.

Regarding the topic of approval, if I remember correctly you don't need actual approval. You have to notify your neighbor that you want to set up scaffolding on their property for a specified period of time within the framework of hammer and ladder rights. I think Allkauf Haus here just needed confirmation that this was notified. Because as far as I know, your neighbor cannot prohibit this. However, it must be properly notified to them and of course you may not cause any damage.

Regarding the legal situation and who is "to blame" in this respect, I cannot say much either - especially due to lack of knowledge of the construction work description and any possible checklists.

But what really makes me angry as an uninvolved party while reading is the approach and lack of communication by the construction company or their site manager. Even if somewhere in the fine print or in the depths of some checklists it says that such an approval or notification might have to be obtained by the client (question of blame), I STILL EXPECT a SITE MANAGER to go through any possibly missing items with me IN A TIMELY MANNER and to inform me that I still have to take care of them. I mean, they obviously KNEW that, otherwise they would have come and built the house.

And then sending a letter two days later while the site manager without a replacement is on vacation - that is really impossible. Allkauf Haus is not a premium provider, but you are supposed to pay them a huge sum. And no, as a client I don’t have to know everything. It is helpful to inform oneself, but if I knew everything, then I wouldn’t need a site manager.

At the same time, I see it like other forum members. Initiating legal steps at this point will probably have a counterproductive effect on the further progress of construction. I would definitely address it clearly, demand better communication for the future, and also clearly express my frustration and disappointment.

It looks like the date for the house installation was scheduled without all necessary measures and permits. The company or the site manager did not say a word about the required permit and planned the installation anyway and officially informed me 6 weeks earlier, even though the concrete slab was not yet there.

I don’t understand: she "did not say a word," but at the same time informed you six weeks in advance – how does that work?

I don't understand: she "didn't say a word," but at the same time informed you six weeks in advance – how is that possible?

Crystal ball ON: He probably means the approval wasn't mentioned, but everything else about the installation date was. Crystal ball OFF:

and explained to me that the appointment was canceled due to lack of approval from my neighbor.

The question is, what exactly does that mean? Did the company send the approval letter to the neighbor and he didn't sign it, or did they realize shortly before the installation date that the process hadn't even been started? In any case, the company didn't get the green light... Only the "vacationer" knows that alone...

I was only informed about the date of the house installation, and that was 6 weeks in advance. The company neither asked me nor the neighbor if they could use the neighbor’s property. I called the neighbor shortly after the installation cancellation and asked if he had received either a letter or a request – but that was not the case.

The current situation is as follows – I have to take care of the approval, and then a new appointment will be scheduled. The whole process will take at least 4 more weeks... So says the "vacationer".

Presumably, you should have been clever enough to take from the construction service description and remember that creating such prerequisites would have been your task. And what was expected from you would then have been the neighbor's approval (which I personally would have rather called a "declaration of consent"). So find out in what form they want that. Then send it and have the receipt confirmed. As a precaution, I would announce an intervention in case the jokers cancel the thing again.

P.S.: Presumably, the vacation substitute simply panicked out of fear of not having the sheet matching his checklist, and "as a safety measure" slammed on the brakes – afraid that the neighbor would say "njet" > the security cannot be provided > the safety coordinator sends everyone home...
(the vacationing person herself probably has the experience that neighbors very rarely cause trouble in such cases).