Mike Rife
Ansys Employee

Hi Lovisa Svanborg

I think you answered your own question.  The material model is not stable as the uniaxial response turns negative at strain ~0.125 and would likely have solution difficulty before that.  But, do you expect uniaxial strains larger than say 0.05?  If not then you could probably use this fit.  Otherwise I'd redo the fit changing the options...maybe use a different hyperelastic material model.