I was able to trigger a FPE at line 102 of usr_rates.f:
RATES(R5) = 6.7d12 * exp(-2.4358d4/T_g(IJK)) * &
EP_g(IJK) * (c_O2**1.3) * (c_CH4**0.2)
My guess is c_O2 or c_CH4 become negative and taking a non-integer power of a negative number triggers the FPE. It is better to only compute the RATES if the concentrations are positive, or above a small threshold to avoid the FPE.