At some point in the timeline, the video reference is no longer visible in the video monitor window (it shows black) when it is in playback, and it only shows the image when it is in stop. This makes it impossible for me to check the sync correctly. Can you please help me to fix this issue?
This has happened to quite a few people, and I haven't yet heard of a definitive fix - but one thing that might be worth checking is that your video card drivers are up to date - this can make a significant difference to this sort of problem.
I have the same issue with Mac and Windows, no fixing yet, did you ever managed to fix this problem??