Solver does not calculate the minimum

[bestox]

i have three different ranges:

BC1:BH1, contain values (f.e. yields)

CI1:CL1, contain some parameter values to start

CO1:CT1, each contains (different) formula using above CI1:CL1 parameters. Formula look like CI1+(CJ1-CI1)*(1-EXP(-0.5/CL1))/....

CN1, contains SUMMYXMY2(CO1:CT1, BC1:BH1)

i now use the solver with MIN:
target cell is CN1
modifiable cells are CI1:CL1

I now run the solver (i did it some more times, but does not change the results, since the parameters are to some extend calibrated). I now assume that the parameters (CI1:CL1) are changed to deliver the minimum of CN1, CN1 contains f.e. 2.1; with simply changing one of the parameters by hand (from -0,04 to -0,05) i decreased CN to 1.7; so the solved parameters by excel are obviously not the best minimum.

I was wondering that maybe i can change the accuracy of the solver. When i enter the options, accuracy is set to 1; When i press OK, i get the messagebox "accuracy has to be a small, positive value". I tried 0.1, 2, 0.5, always same messagebox.

anyone has an idea how to retrieve better results? what could be the reason for the bad parameters?

Thank you for any advice.