Somebody else had a similar problem quite a while back at least twice.
Rick fixed it.
I recall it had something to do do with someone editing the original post.
But my
memory is not as good as it used to be.
Well the other user was Outdoorking he had a few posts on this but I don't see a resolution. Other than possibly the thumb image file was deleted somehow.
So maybe you need to open a support ticket to fix it. Or at least find out why it is happening.