Great. It seems the same issue also applies to the XNA runtime. Still investigating here though.
So I fixed it. First the file is again missing.
Second: I changed SkeletonMeshRenderer.cs Line 93 to:
var drawOrder = skeleton.DrawOrder;
And Line 96 to:
Slot slot = drawOrder.Items[i];
Also SkeletonRegionRenderer.cs
Line 86 to:
var drawOrder = skeleton.DrawOrder;
And Line 89 to:
Slot slot = drawOrder.Items[i];
That fixes everything for me.