hi, I have exactly same problem but in my case the ERROR URL says below
ERROR: The fullsize image file cannot be found in your uploads directory at 2020/08/The-burning-monk-1963.jpg. Without it, new thumbnail images can’t be generated.
now the fullsize image is available here https://www.wikigrewal.com/wp-content/uploads/2020/08/The-burning-monk-1963.jpg
I have tried both REGEN THUMBNAIL & FORCE THUMB plugins…. same error for most of my images.
Tried deactivate,uninstall,reinstall.. no benefit.
Pls help
