How did you get the Combined time hardening coefficients? Via APDL Material models? You have fixed C4=0, so I assume you don't have temperature dependant model.

Try to fix C2=1, C4=0, C6=1 and C7=0 (fixed stress and no temperature dependancy) in the Curve fitting tool and try to run the simualtion again. The creep behaviur curve should be nearly the same (you can compare to the current one that you have). Put fixed values in the Coeff Value, Fix them and click Solve to get new values for the creep simulation.