Jake, it looks like you are missing (or overwrote) your /includes/config.inc.php file with the stock one.
The error message means that its looking for a file specified at a configured directory listed within that /includes/config.inc.php file.
If you happened to accidentally overwrite your forum's config.inc.php file, you can either; A) rename and use the config-bak.inc.php file that UBB.threads.7.6.2 created within that same /includes directory. or B) restore an earlier version of your config.inc.php from the backup you did just before performing your upgrade.
