I have read the Focus article. Not particularly enlightening.
the investor buys your loan and calls it due for dubious reasons
Banks/investors can only call the loan due or auction it off if installments have been missed. Or after the end of the fixed interest period. It is clearly regulated by law how many installments have to be overdue for how long before that is even possible. Being a little late once is not enough.
You cannot terminate the contract for minor issues, such as forgotten information to the bank.
Unfortunately, immediate termination is possible in case of imminent danger of the borrower's insolvency, but that is all very abstract.
Here Focus could have written more than just general blabla.
That is what happened to the neighbors
I am very interested in that, can you elaborate? What exactly happened, why was the contract terminated, and why didn't they find follow-up financing? Was there maybe unemployment, massive devaluation, or other disasters?