I periodicaly have this problem on newgrounds.com where by I cannot log in.
I select sign in, the window pops up, I enter my password, it says welcome.
But when I try to do anything on the main site the site reports that I am not logged in and that I really should log in.
Also, the number of votes a day I do do not seem to add up. It used to be that they did even if I could not log in, but as of around 4 or 5 months ago even that stopped working (may have been only 2 or 3 months ago, my temporal sense is not very good.). Before after 5 votes I could deposit my coins and my account would be credited, but before the change over to the new UI it would say "thats 1 animation you have voted on, four more to go!" no matter how many animations I had voted on.
Now that message has seemingly been removed from the new UI, so I just get a "Why don't you sign-in!" message instead. Which is pretty useless, since, err, heh, the system will not let me sign in. :)
This problem presents itself in both IE 5.00.3315.1000(sp2), and in whatever the latest release of Mozzila was when I downloaded it sometime either early this month or more likely sometime last month.
Hmm, ~2 gigabytes free, should be enough for a cookie, so no, that is not it. :)