I use Google Chrome and I have it on "Remember Me" so I don't have to go to the trouble of typing my username and password again and again. Which browser do you use?
I am having similar issues. I use Internet Explorer 8 and immediately after logging in I get an "Invalid redirect URL" message. I then try again and it says that I'm logging in and then immediately logs me right out. I have it set to accept all cookies from this site. After multiple tries it finally works.