It seems likely that AB's server can't find the necessary cookie on your computer. Either:
(a) your security settings are too high, so that your browser isn't storing cookies ; or
(b) your computer's clock is set to the wrong time or date, so invalid cookies are being created.
that should made a difference, rocky, 'private browsing' is supposed to stop the next person sitting at your computer being able to call up the sites you've been on.