Can the move-in be postponed due to defect remediation by BT?

It always depends on what was agreed upon. So first of all, read the fine print and, if necessary, inform the [GÜ] about everything beforehand.

With my construction, I was practically allowed to do whatever I wanted and had access to the construction site from the beginning and later even had a key; it was also no problem to bring in experts, etc.

So, as I said, read and clarify beforehand.

read the fine print, exactly right ^^

this is about a property developer, not a general contractor :) with a general contractor, you can theoretically do whatever you want on your construction site, since it is your property. no one can refuse to send an expert, because as the owner you always have the right to direct on your property. but this is about a property developer, and things like payment are not agreed upon in the fine print but clearly stipulated according to the real estate agent regulations?! or am I seeing this wrong?!

If it is a BT, for laypeople they are all BT's anyway

It will probably be a bt, as it concerns a multi-family house (3+ units).

Is the developer allowed to postpone the move-in date communicated by email (we have terminated our apartment accordingly) to the original move-in date according to the purchase contract (which is 2 months later) by remedying the following defects?

I believe you need competent advice (--> specialist lawyer for construction law). Move-in date is the wrong term. After acceptance (= taking possession by you), you can move in whenever you want. Immediately afterwards or 1 year later, it doesn’t matter. If there is a date for acceptance (possibly a formal acceptance is agreed, have a look at the contract), then the developer will probably insist that you also accept. Because then he is off the hook. If there are serious defects that oppose acceptance, then you are allowed to refuse acceptance. And he has to rework. If there are no such defects, you have to accept. And then you are allowed to move in.

Then there can be defects that are less serious and thus do not oppose acceptance. Then you have to accept, but may still not want to move in because craftsmen will come again for the rework and cause dirt.

In any case, a specialist lawyer will surely be a help to you ... Call him, ask what it costs to review your contract and to accompany you for the acceptance.

I always only read specialist lawyer... man, the first acceptance was free of defects, so there can only be defects that a layman did not see or the expert sees. Hidden defects that are bigger are almost impossible (?)

Just ask your BT directly. Simply call or write an email what happens then and then

Why is there always fear of it? It's legitimate. We had such a good relationship with our BT, we exchanged information and the site manager knew that we had to terminate an apartment