Problem with 4 parameter logistic fit for immunoassay

Sure you can find a fit using a solve block with minerr and ever so often this has some advantages over using the built in "genfit" function.

But I doubt that the approach as posted by you ever worked in older Mathcad versions. We had to see the pdf to judge though.

Here is an approach using "genfit":

Please find in the attachment another approximating model for your data.