I'm seeing something different. The SkeletonDataAsset isn't being generated at all.
So what you guys are saying is:
Even without using Transform Constraints, if you export with 3.0.11, importing in Unity doesn't work? And exporting with 3.0.10 or earlier works fine? Can you share some jsons we can compare between the two?
Wait, were you using weights and meshes and didn't update your runtime in the past 3 days?