Hello troberts
Thanks for the help.
Unfortunately I am still getting the same errors.

I have attached here a .rar with the base .fsp file that I use, the Python script I run, and the initial Forward and Adjoint simulations. (I can't attach the .fsp directly!)
I have checked throughout and am using a 40nm mesh.

Interestingly, if I include an "initial_guess", and have inital_cond=None, then I get a different error about the bounds of the parameters...