Hi,
It looks like there is an issue with the skeleton itself and not the engine. See in the Output Log:
LogFbx: Warning: Not valid bind pose for Pose (skinCluster2) - Node Doodads : The relative matrices do not match
LogFbx: Warning: Not valid bind pose for Pose (skinCluster2) - Node BadassSpider : The relative matrices do not match
This would be a starting point.
-Thank you