Taskbar thumbnails show blank squares instead of live previews

Lately, hovering over taskbar icons shows empty white squares instead of window previews. It works sometimes, then stops for no reason—restarting Explorer brings it back briefly, then it breaks again. I've already disabled and re-enabled thumbnail previews and cleared the thumbnail cache, but nothing has stuck.

Anyone know what causes this? Would love a permanent fix instead of restarting Explorer every hour.、