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.)
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