I saw that "special case" previously posted this identical problem. The response by pixelmage was this:
"Silly question, but is your Canvas showing you the alpha channel (instead of color)? Press Shift+C to force the view to the color (RGB combined) mode. Does this alleviate the problem?"
Special case never reported whether or not this was the problem. I am have the same issue, however, and it is not the channel view setting. I tried all of them. All of the settings display a white screen except for RGB,Red only, green only etc., which make the preview window go black.
If I do a Ram preview, then I can see the video... huh? I'm totally baffled.
Just yesterday, I was playing a project with many layers, sound etc.. and it played with no problem. I am playing out to my desktop.
All I can think is that i recently ran software update and updated apple studio.
Ok.. I had a nice nip and figured out this silly problem... if this was special cases' special case, perhaps s/he felt too silly to admit it. I have two monitors. I had recently moved my computer and scrambled up which monitor was my main one (the one attached to my Radeon)... soooo, when I moved the preview window to the other monitor, voila, I'm back in business.. thank you Tanqueray, you make a fine gin and tonic and a whole lot of sense.
Glad to hear your gin-and-tonic therapy method worked :) It was pixelImage that was having the problem: I had responded, trying to help. Yeah, it looks as though Motion can only draw images in the Canvas on your primary display (the one with the menu bar). I would guess that the RAM preview worked because the images were cached in RAM, first, instead of coming straight from the GPU.