Custom mesh gets messed up during retargeting

Hi there! I’m currently working on a game with a few others that features custom animations with a custom skeleton and we hit a snag; the mesh + rig + animation looks fine in 3dsmax, but in unreal, after they’re retargeted,they all look like this.

I’m stumped, the rigger is stumped, the character modeller is stumped, everybody is stumped. We got more stumps than a deforested park. Does anyone know how to fix it, or if there’s anyone who’s probably willing to fix it? Any help is appreciated!