So after a day of testing, we've come to suspect that Anima is the culprit. Rendering with motion blur over Backburner hangs on parsing, rendering locally only works when Anima isn't included in the scene or when rendered by itself. So there seems to be some sort of conflict there. There is also a zero scale error message that pops up some of the time for anima which we suspected was the cause, but this is also really hard to pinpoint. We've attempted many isolation tests with anima alone, anima with a simple xref (empty scene) etc and still can't render.
