As I said above, you restored your UBB.threads folder and re-uploaded files; you mentioned:
I uploaded only the flies/directories listed in the upgrade guide.
Again, what files did you
NOT upload, as all files in the archive are required (even the language files, which where updated quite a bit from v7.5), with the exception of /includes, /cache_builders/custom, and the /styles/wrappers.php file.