Chuck, I've upgraded this script a few times now. I know the process. As I said, though I don't think I did anything wrong, I reverted to a backup. I've uploaded the previous good files. I've checked the config file 20 times to make sure it's pointing to the correct directory. I've checked phpmyadmin to make sure the database settings are pointing to the correct directory. I've tried manually changing the database prefix in phpmyadmin - doesn't change anything.
The config file is pointing to the correct directory. What do I try next?