Hi there Sketchfab Devs
I was trying to import a skinned mesh where just one of the bones is using a different rotation mode , the default for all other bones is Quaternion wxyz , I tried both Euler xyz and Axis angle for this one bone , it works perfectly in Blender, I can also reimport that FBX into a new file and it works as expected.
In sketchfab this bone does not rotate properly , it is supposed to spin around its y axis only .. but instead it gets all wobbly on the other axis' as well.
Now what is strange is that I can use the blender addon to upload the model and it imports as it should ( well 99.9% perfect , small bit of imprecision but I could get around that problem )
Im not sure if you are aware of this issue , I can provide the fbx files for testing or the blend file etc..