20-year fixed loan at 1.7%, who is behind it?

Did you enter your exact conditions (loan amount, equity share)? All the offers usually only apply from 50% equity, if you didn't vary that the interest rate – but it will then also be displayed accordingly.

We have 40% equity. Online, 1.54% came up. After a telephone conversation/consultation, it was 1.86%. The KFW loans 153 and 124 are subsidized at 0.3%. I have all of this in writing. But it is uninteresting for me because I get the same conditions from 2 other lenders where I also have a personal contact person on site.

Online, KfW is not included. If you run a part via KfW, the share financed through the bank decreases – and thereby also their profit (interest). Enter your actual equity + the amount financed through KfW as equity in the loan calculator. Then you will probably be at only 30% financed through the bank. That makes the loan expensive again. Banks finance most cheaply at about 50% equity if the rest is fully financed through the bank.

283,000 loan amount.
of which 183,000 bank
and 2x 50,000 kfw
I consider all of this to be bait offers on their website

So I once received exactly the offer shown to me there from Dr. Klein. These are all just proposals, so ideal values - in reality, you can be off by a few percentage points.