If MySQL users were messed up, your web interface wouldn’t work. FOG doesn’t setup a ‘fog’ user for MySQL. Typically it uses the root user without password to access MySQL if you’ve not manually defined a user/password in MySQL.
I doubt it’s permissions. It could be a corrupted image - but if that’s the case, it’s likely that the server HDD is dying or underwent a sudden power loss.
What’s stopping you from making a new image and trying that?