Changing opacity gives me a green screen of doom

I'm having a problem this morning, and searching for answers is almost impossible on this one.

I'm trying to create a video overlay effect. V1 is my untreated video. I copy that, paste the copy directly over my clip on V2. Using the basic motion parameters, I scale up the copy on V2 & turn the opacity down to 40%. When I turn the opacity down, instead of seeing the layer underneath, my frame goes completely green. Scale & center don't cause this, just opacity.

What's going on here? Is this a "trash the preferences" problem, or something else?

Jeff

What codec are your videos?



www.strypesinpost.com

ProRes 4444.

Footage originally came from a Canon 7D. Knowing what I know now, I'd have transcoded to ProRes 422, but live and learn.

I've done similar effects with the footage in the same timeline earlier in the edit. This seemed to crop up suddenly.

Sounds like your computer can't keep up with that much info. If you open a fresh project all by itself and do exactly what you were just trying, does it work ok?