You are very welcome,
Like you, I would have thought that you would populate both black and both white slots. However, if you look carefully at the System Board diagram and lables, it shows Slot 1 and Slot 2 (white and black) as Channel B and Slot 3 and Slot 4 (black and white) as Channel A. This tells me that you don't populate both black slots, but one white and one black. I suggest that you try populating Channel A with one module each to see if that works.