Video "blackout" on 1 of 5 participants

Today, 1 of 5 participants in a meeting I hosted was blacked out, with only his name appearing.  I'm certain his webcam worked fine as he just came off of 3 previous meetings. He's been on dozens of my meetings, and I've never had this issue with him or anyone else.

Is there some setting causing this? (And yes, he "started video" on his screen, attesting that the red slash had disappeared.)  Suggestions?