Taskbar thumbnails stop showing previews and just display blank squares

These are previews of running application windows, not the saved image thumbnails in File Explorer, so clearing the file-thumbnail cache would not necessarily address them. Windows uses Desktop Window Manager for window thumbnails. First check whether the blank previews affect every application or only one, and whether Alt+Tab previews fail at the same time. Compare a simple Notepad window with the applications where you first noticed it. If only one application is affected, update and troubleshoot that application before changing Windows globally. If the failure is system-wide, check the display driver supplied for your computer and test a clean boot using Microsoft's procedure to isolate background software. Record anything disabled and restore normal startup afterwards. Explorer restarting briefly helping is a useful observation, not proof of a particular cause. If the issue remains, submit the Windows build, graphics-driver version, affected applications, and a screen recording through Feedback Hub.