Did you take the 180 deg shift into your account ?
Since your measurement was done when the loop was closed, there must be an additional 180 deg phase shift (in other words, minus sign).
In the end I just fitted the response magnitude. I was initially fitting the complex response function, but ran into problems which I think were cased by overall phase offsets between the data and test function. Can I canvass for opinion if fitting the magnitude is OK, or should I try again fitting the phase too?