This probably still isn’t relevant to you (as two years have passed) but I was having the same issue on my mid 2011 macbook pro (running Motion 5, not updated from 4 to 5). There aren’t any solutions that I have seen yet, but if you restart your computer and try exporting again, there’s a higher possibility that the white flashing glitch will not occur. I recently have been exporting 3-10 second clips, and sometimes i have to restart my computer twice or three times before it exports without any hitches. Hope this helps anyone else.