I have two ATV2 systems - one is currently on 5.2 with ATV Flash 2.3, and the other on 5.3 untethered with ATV Flash 2.4.
I have some ISO’s that give this problem you mention when I try to play them off my NAS - but that happens on both the above ATV systems. Other ISO’s play fine on both systems. That means any issue could be specific to some ISO’s rather than a generic problem affecting all ISO’s, so it could be worth raising a support incidence for your problem ISO(s).