I'm experiencing a similar problem. I registered last week just fine, but when I came back today I had to log in again and couldn't. The bottom of the page would periodically pop up the note saying I need to accept cookies, which I did each time. Every time clicking on the login link at the top of the page or within any forum thread, I would get the popup saying, "Oops! We ran into some problems. Cookies are required to use this site. You must accept them to continue using the site."
I got around it, to post this, by using a different browser. My main browser is Firefox, and it did receive an update in the past day or two, so there's probably something incompatible between the latest version and the forum's cookie detection code.