That's got nothing to do with cookies - that's a known threads 6.4.2 bug - if you want to fix it yourself, you'll find the fix posted here:
https://www.ubbcentral.com/boards/showflat.php?Cat=&Board=UBB5&Number=36553


Joshua Pettit
Web Developer
www.ThreadsDev.net | www.JoshuaPettit.com