The TDR forum software was upgraded right about then. Whenever that happens there are problems like this with cookies on pcs and them not recognizing or releasing them, etc. We've dealt with this many times over the last 17 years. Please trust me and try the directions I linked above and I can almost guarantee your problem will be solved. I could go back and show you at least 100 posts where people doubted this would solve the problem and when we finally convinced them to try it that the problem was solved.
Guys, it takes like 30 seconds to try the things suggested above. It's taken you longer to respond to this again with your doubts. The cookie has hash settings in it to key it to the site. When vbulletin upgrades sometimes they change how the hash is calculated. If your browser doesn't allow the old one to delete you'lln have the exact problem your having. I've had it happen to me too. EVERY time it's happened deleting the cookies for the tdr and then logging back in with remember me checked had solved the problem - as it has with all of the users that have done the same.
seafish - I started having that same problem with Safari on 2 other forums I take part in over the last 6 months or so - although not on the TDR. I haven't been able to figure it out - both use different software, btw (neither uses vBulletin). No idea what's going on there, if I figure it out I'll let you know. I also run windows in a VM on my mac and don't have the problem with those same sites using IE or chrome in windows.