visco-plastic strain rate dependence material umat

[malordo]

Dear all,
I'm working on a material model with strain rate dependence, I found a very useful subroutine in the literature, that I modified for my purpose, and it is actually relatively working fine (it depend of the elastic modulus), it doesn't use the effective Jacobian, for high values of elastic modulus the results are not correct. My question is: can it be related with the simplified Jacobian formulation?
I'm attaching the original umat subroutine.

Thanks in advance