Update PHP to 5.4.x and you will have the problem. I assume that's what UBB Central is running. I can confirm that I am running 5.4.16, and I see this problem, and deleting the cookies fixes it. (I just did one yesterday.)

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