Still having the problem here too (using Safari browser). Deleting cookies doesn't fix it. I already gave detailed info to GiantDomains, and created a ticket.
The problem seems to happen only on DNForum, and not on other vBulletin boards, so I suspect some DNForum mod/add-on/customization is to blame.
I was looking at the individual cookies (via Safari's Preferences-->Security menu), and comparing them to other vBulletin boards. Other vBulletin boards seem to set a "bbpassword" and "bbuserid" cookie. However, DNForum doesn't?
Also, DNForum sets __utmc and __utmb cookies, however their "Expires" are blank and today, respectively (maybe one of those or both should be a different expiry length?). The bbsessionhash cookie also has a blank expiry.
It could be that Safari interprets a blank expiry different than other browsers?
The full set of cookies that I see being set by DNForum are:
.dnforum.com sets: bblastactivity, bblastvisit, __utmc, __utmb, __utmz, __utma, bbsessionhash, bbthread_lastview
and
www.dnforum.com sets: IDstack, leaderboard_domains
That looks like 10 cookies in all.