Attack index is one of the two attack types a unit can have. For example, if you've seen how melee heroes can attack air units at range when they have an orb, that's because their 1st attack is melee and their 2nd attack is ranged and can target air, and the orb is set to enable attack index 2.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.