You might be able to try recalculating the roll of the bone responsible for this motion based on the local +Z axis. I had a similar problem and this fixed it. It will probably mess up the animation in blender though. I would recommend saving a backup copy of the original in case this goes wrong.

I had a similar problem with my custom player model. What I did to fix it was to select the bone responsible for the weird deformation and recalculate its roll using the local +Z axis. The model animated fine in the model viewer; however, it now looks weird in Blender when using an exported animation from source (ragdoll still looks fine).