You didn't follow this step in the Upgrade Docs did you?
Upload all files and directories from the unzipped UBB.threads™ directory to the UBB.threads™ installation directory, except for the includes, styles, and cache_builders/custom directories. You will want to upload all of the .php files within the cache_builders directory, just make sure not to upload the custom directory or files within it.
Happen to have a backup of your forum directory so you can recover the files in those directories?
