If you take a look at the formula for calculating current (I), you will easily understand the temperature could affect the exact calculated current value. I am wondering why your calculation for T=1000K was finished quickly. I doubt the charge may be converged to zero. You had better examine carefully the corresponding log file or plot the transmission spectra for T=1000 K, 400K, and 500K in order to figure out why the current (I) is close to zero.