I think this is because we are only meant to have Tier 1 missions at the moment, so any attack or defense rating above that causes the missions to give negative credits xp and fate.

Not 100% sure but i think you (along with others) having access to gear that puts the stats so high missions are unplayable is a bug. We are only meant to have access to the Tier 1 gear to use in the current Tier 1 missions.