I also suspect this is being caused by a mob-related mod, most likely Bad Mobs... or perhaps a dodgy BiomeTweaker script?
Check the Bad Mobs config and any relevant BiomeTweaker scripts and look out for any 'max' values that are lower than (or equal to) any 'min' values, or any 'min' values that are greater than (or equal to) any 'max' values.
Wonderful help. Thank you very much. Found an invalid value within the config file for the Animania mod. They don't allow a value of 0 for spawn rates or frequencies.