Have you tried flushing browser cache, different browser, etc? If not, try holding down 'shift' while clicking reload button in Firefox.
What firmware are you coming from? If it was earlier than 3.1 then there are drastic changes in the webUI and its supporting files. (javascript, etc) Any of those files cached by the browser could potentially derail the login.
I think I found the root cause. When I try to web login with a hostname (xxx.yyy.lan), it doesn’t work. But when I use the IP address, I can login without problems. Could you try to web-login via IP address?