Been getting a few of these today.
Same here, I’ve been having difficulty logging in all day long today.
It's the age old cookie problem. On the old site after logging into the forum and some time later visiting the main site I would get logged out from the forum. Annoying because the reason I would visit the main site was to look up data to reply to a post that I was invariably half way through posting, and loose the reply I had already typed.
On the new site it seems to log in fine but after some time if I log out and back in it complains I don't have cookies enabled and brings up the captcha challenge. I found the best work around is to open a private window in firefox so no cookie data is remembered when I log in/out of the site and close the window. This would also work if I do clear private data but then I loose other sites I visit and want to keep tabs on.