OK. This is clearly my problem. I need to fix the webserver so it will display that directory. Thanks for all the suggestions that helped me focus on where the problem actually is.

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

Server Information
UBB.threads Version 7.7.5
Release 20201027
Server OS Linux
Server Load 0.16
Web Server Apache/2.4.6
PHP Version 7.4.33
MYSQL Version 5.7.43
Database Size 1.85 GB