It sounds as if your browser is set to not accept cookies. You may have had PC on an exceptions list ("accept cookies from these sites...") but gotten rid of the exceptions list during your housecleaning.

I'd say, start by making sure that forums.psychcentral.com is on your browser's list of sites to accept cookies from (if you're blocking most cookies) and not on the list of sites to refuse cookies from (if you're accepting most cookies).

If you have more than one browser available, you might want to check if some get you in here more easily than others. That just might help you figure out if/how the cookies are being treated differently.