Vegas 21.0 build 315, Windows 10 Pro, Radion R9 200, 16GB RAM, Intel i7-5280@3.3GHz
One video track displays just its upper left corner in the preview window. It is correct on the track and crop window (see screenshot), and the rendered video. The other tracks behave normally. All videos are the same format. 23.976, 3840x2160x32, Apple AVC. Restarting Vegas has no effect. Suggestions? (And yes, I know I need to upgrade my hardware.)
(Just FYI, I had no control over the originals. They were shot in Bulgaria at the premiere of my Symphony No. 4, so I was just given an HD to take home with all the audio and video files.)