I've been having the same problem for the last few days, using Safari on my Mac. Each time I try to access a new page on the RS website, there's about a 25% chance that I get bounced back to the login screen.
Very annoying -- but it's reassuring to know that I'm not the only one that is affected, and it's not a phishing-type scam trying to lure users into revealing their passwords.
Update: I've deleted cookies (etc) associated with ratesetter.com and that seems to have resolved things - for the time being at least...
The only way I can login either on my phone or my laptop or my Chromebook is to use 'incognito' mode in Chrome and then I get in. Otherwise my login attempts just bounce my straight back to the login page ad infinitum.