Found the post. I replaced the libs/html.inc.php file and that fixed the problem. (Tested to confirm the fix.) SirDude mentions two others; header.tpl and adduser.inc.php. Were there more bugs fixed silently? The download still has a 11/29 date on it, but it's clearly different from the one I pulled on 12/5.

The Stovebolt Geek
https://www.stovebolt.com/ubbthreads/ubbthreads.php

Server Information
UBB.threads Version 8.0.0
Release 20240826
Server OS Linux
Server Load 0.11
Web Server Apache/2.4.37
PHP Version 8.3.11
MYSQL Version 8.0.39
Database Size 1.82 GB