Why green playback of video clips?

Imported iPhone 6 video (shot vertically) into Photos. Using iMovie to edit them into a project. When I add them to a project timeline, they play back as full-screen green, with the audio playing fine.

I've attempted to add a stabilization effect to force a re-render, which has met with intermittent results. It works fine for a while, but as soon as I quit and restart iMovie the next day, the green playback returns. Green remains even when shared out to YouTube or exported to the desktop.

I find it difficult to believe that every shot I recorded with my iPhone was corrupted.

Bill