Galvanic Cell: Temperature vs Voltage
Indeed, it should be inversely proportional. In case you divide the copper concentration by the nickel concentration, the sign of the RT/nF part should be plus and not minus. If you divide the product of the reduction by the precursor concentration, you have to use minus.

The temperature has a great effect on the current, especially in the "Cottrell-determined" area. The Nernst equation, which you use in this case, is true for a near-equilibrium environment with fast electrode kinetics. If you drive the potential too high, mass transport can become so slow (in comparison to electron transfer) that an increase in potential will not yield a higher current flow as predicted by Butler-Volmer kinetics.