Same problem. Yesterday, I signed in, made changes to my site and then published the edited pages without any problems. Today, after making additional changes, I get an error after clicking the publish button saying that I must sign in first. Obviously, I had already signed in in order to edit the page. Could there be a problem with the page itself or some sort of bug?
Using the web based SiteBuilder Plus via Firefox on a Mac.
The most common reasons for that error message are due to a period of inactivity, where the website logs you out for security reasons, or if there is a break or small hiccup in the internet connection. It is most common with dial-up, satellite and wifi connections which have weak signals or services that do not provide a constant on connection.