I have a similar case and came across this thread here.
I am not a fully qualified lawyer, but I work in the legal field, so I am quite familiar with legal matters.
The legal core question in this matter is whether the granting of the forward loan is considered an independent new loan agreement or merely an amendment of an existing loan agreement. If it is just an amendment, the normal 10-year period applies. If it is a new loan agreement, then the possibility of termination would only be available 10 years after the end of the forward period (i.e., 10 years after the start of the effective loan term of the forward loan), because the formal disbursement (in the legal sense) only takes place in the future and only then does the period begin to run.
With the relevant passage of the Building Code "after receipt of the loan, a new agreement is made about the repayment period or the nominal interest rate," the legislator's intention may have been to aim at the fact that in the event of payment difficulties with the installment, the bank can agree with the borrower on a new term, a new installment amount, and a new interest rate (similar to what happens in the case of debt restructuring, for example), and that the 10-year period does not start to run anew with this agreement. If, in the case of the forward loan contract, not only the interest rate but also the repayment rate is changed, this speaks, for example, rather in favor of a new loan agreement. For interpretation, it is always helpful to know the legislative history of the legal text. That is, what were the intentions of the legislator? This is considered by good judges.
I am also aware of legally binding court decisions according to which termination is only possible 10 years after the start of the effective loan term of the forward loan contract (here in 2024). In practice, judges often refer to prior case law from other courts, provided the case is comparable in principle. However, this is also no guarantee; the individual case must always be considered. (For example, is there a transcript of the consultation with the bank on the part of the borrower, where the bank explained that termination can already take place directly 10 years after the forward loan agreement was concluded, and/or can the consultation be confirmed by a witness? etc.) Because the borrower does not have to be able to understand the underlying laws. It is sufficient if he relies on what was (possibly incorrectly) said to him by the bank (and he can testify to this).
In any case, this matter here is definitely anything but clear, even if some self-proclaimed "experts" claim otherwise!
I tend to agree more with the financing broker’s view that the longer period applies. In any case, there is an immense risk of winning any potential lawsuit. I advise against it.
What I would specifically do in this case:
I would present the bank with the view that termination is indeed possible 10 years after the forward loan agreement (possibly one could also bluff additionally and claim that a lawyer gave this information). Now, if the bank insists that termination is only possible about 3.5 years later, I would basically try to reach a pragmatic deal: for example, paying the high interest rates until mid-2022 and then continuing with a new, lower interest rate, in the spirit of "half and half."
I would always only pursue a lawsuit as a last resort and especially only if I were sufficiently/nearly certain to win it, which, however, is not the case here.