I have received this with the free download section if I take too long to enter in the 3 characters and start the download. There is definitely a timeout and if I get bored waiting the 45 seconds and start doing something else, the session often expires.
Alternatively, it may be a browser/ISP/proxy problem. Rapidshare says to change your browser, disable the proxy server (if you can and if you know what it is), or buy a premium account.
By the way, the premium account really is worth it.