I have uploaded a model to mixamo.com. In mixamo, I've applied an animation, and in the mixamo preview window, the feet are perfectly stable, they don't skate at all.
I have downloaded the resulting fbx animation together with the skin, and I have opened it up in Autodesk FBX review.
In Autodesk FBX review, the feet are also perfectly stable.
I have then imported the fbx in Unity without changing anything.
In the animation preview window, the feet are skating.
Why is that so, and how do I get rid of it?