Thank you for checking the permissions in Security, that rules that out as well as clearing the cache.
Have you tried to rebuild the Preferences?:
Quit Bridge (if running). Just before clicking on Bridge in the Dock or double-clicking the application, for the Mac, press Command-Option-Shift. For PCs, press Control-Alt -Shift. Then open Bridge BUT KEEP your fingers on those keys until you see a window pop up. From the window, choose “Reset Preferences," "Entire Thumbnail Cache”, and/or "Reset Standard Workspaces.” In your case, choose "Reset Preferences."
Have you made any adjustments to the jpg images using ACR?
What happens when you open these images, do they look fine, or are there image issues?
Is there an image you could share to see if anyone else is having the thumbnail displayed?