In several places phpList adds a secret code which can only be used once, for security purposes. If you get this error, you need to reload the page, in order to get a new code.
Make sure to reload the page with a “GET” request. If your browser asks you to confirm re-POSTing the data from the previous page, you will receive this error again.
Use the menu to browse back to the page you get this error, and try again.