With your help, I was able to render correctly. I reset the Project PAR to be the same ratio as the videos inside it. Then when I render, I set the PAR to 1.0.
Maybe that’s the issue – as John said – that it doesn’t like oversized still photos. Some of mine are 3072 x 2304. They are only links to the files though – not saved inside the project. I’m surprised it’s a problem, really.
I have still images about 2mb each on the timeline – but on a separate track. I’m working on a project now with two 3MB still photos and one short vid, and I get the same response. And it doesn’t make the still images red, just the vids.