[Daniel Johnson] "If you watch this slowly you will see that the seams in the box are visible due to the motion blur."
Much ado about nothing Dan cos folks won't and aren't supposed to watch it slowly. This is a classic case of solving a non-problem. At normal speed it looks fine.
But if you want to be pushy about it try applying motion blur to the cube as a whole instead of applying motion blur to the individual layers that make up the box.
If you were to pre-render the box without motion blur and applied CC force blur then you should be fine. The issue comes about when motion blur is applied separately to individual layers. In such instances one layer may inherit a different motion blur setting from another. Thus you get the artefact along the edges.
HTH
RoRK
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