Subsequent demand for securities by the bank?

and how is the additional security supposed to look if suddenly all houses in Germany are worth only half?!

The last ones get bitten by the dogs. This will vary greatly by region. And even if house prices fall by 50%, a lot of people are not affected. All those who started with a loan-to-value ratio of 60% or have been financing for a longer time and have practically repaid enough.

When banks switch to panic mode, it’s only about risk minimization.

One possibility will be to increase the repayment rate.

As I said, we are only talking about the cases where enforcement would no longer largely recover the RS.

It's not as if something like that hasn't already existed somewhere in the world. About 10 years ago, somewhere far in the West.

The value of the property is only really interesting to the bank when the loan is no longer being repaid.
Why should the bank forcibly auction off a secure loan if the loan is always paid on time?

Here's an example:
Loan amount: €350,000
Value of the house at purchase: €400,000
Value of the property due to boom: €200,000
Outstanding debt on the day of the boom: €320,000

Sale proceeds calculated through forced auction: €150,000
Loss for the bank: €320,000 - €150,000 = €170,000

Further losses due to immense disputes in court. They will never see the outstanding amount of €170,000 because private customers simply file for insolvency (have to). High loan without collateral = hopelessly in debt

The loan was always paid on time monthly. If the bank cancels the loan, the medium-sized business is driven into insolvency without reason. That, in turn, could be a great opportunity for the court, and the state would have to put a stop to it (just like back then with the sale of loans and their cancellation).

In summary: about 50% loss of the loan, simply self-inflicted. If the loan were simply allowed to continue, money would continue coming in, and the loss on default would become smaller month by month. If the loan did not default at all, that would also be a win for the bank.
Conclusion: the bank has absolutely no interest in canceling the loan due to a loss in the value of the property. Their interest lies solely in the loan being repaid.

It only gets difficult with a follow-up loan at the end of the fixed interest period.

With extremely high property losses, I see completely different possibilities for cheating. The old owners stop paying, the auction takes place, and a relative buys the house cheaply, while the old owners file for insolvency. In the above case, €170,000 (- insolvency money) saved just like that. Of course, the requirements are high, bla bla, but for €150,000 they will come up with something.

The whole thing is theoretical for now anyway.

No bank will make a fuss over an additional 20,000 euros in blank. Of course not. But if it’s an exposure with 5 properties (each 300,000) and 80% loans, it looks much gloomier when there is a decline in value.

But honestly: such things rarely come alone. At the same time, the unemployment rate rises, etc. pp.

The value of the property is only really interesting for the bank when the loan is no longer being repaid.
Why should the bank foreclose on a secure loan if the loan is always paid on time.

Here’s an example:
Loan amount: 350,000€
Value of the house at purchase: 400,000€
Value of the property due to the boom: 200,000€
Outstanding debt on the day of the boom: 320,000€

For the bank, the value of any property is practically interesting every quarter. After all, the bank has to conduct proper risk monitoring.

And if the key figures indicate that the outstanding debt is less than the recorded value, this will be reflected differently in the balance sheet and the underlying risk structure.

In any case, the bank will never wake up and suddenly have 200K sitting on top of 320K. By then, you would have already been spoken to several times.

The bank's main concern is initially to have leverage to force you to the table to talk with you.

What they do then is a completely different question and depends on many factors.