i dont think it's a full-out inability to read body language, i think the problem is reading NT body language.
this is becouse with my ASD friends, i naturally feel non-verbals just like NTs describe their bodylanguage, but when i'm around NTs, i cannot read theirs, just at they cannot read mine.
i have, however, learned to deduce the meaning of nonverbal cues by observing the reactions given by others in the group, even to the point that i almost instictively know the more common ones without any external help