You need to first make sure that you are completely logged-out, before trying to log back in. You should click on "Log Out", at the top right hand corner of the green bar. After you log out, log back in using the "remember me" option, and you should be fine... There's also a new forum that Simon has created. Here's a post that essentially describes the same process:
I can't post to the site and I've completed registration
Best regards,