Taskbar thumbnails stop showing previews and just display blank squares

When I hover over taskbar icons, the preview thumbnails show up as blank squares instead of live previews of the windows. It happens randomly—sometimes it works, sometimes it doesn't—and restarting Explorer only fixes it for a few minutes.

I've already disabled and re-enabled thumbnail previews, cleared the thumbnail cache, and checked for updates. Nothing stuck.

Anyone else seen this or know how to fix it?