Copy animation from one rig to another (slightly different)

Dear all, old time blender user, but new to animation here…

I’m trying to copy some “home made” motion capture data from one rig to another by copy-pasting all keyframes in the Graph editor. The problem I’m encountering is that the arms of the target rig are animated in a weird backward fashion and I can’t figure out exactly what’s causing this inversion.

I have noticed that the roll value on the upper arm bones are different, but changing that does not seem to fix the problem (or maybe I’m doing it wrong). Can someone help me figure out what’s going on?

.blend file here:

Thank y’all already.