strange sessions

Hello, I'm experiencing a weird thing regarding sessions - that I'm using for user authorisation. I have a page that the "public" can view (index.php) that includes a login button. When the login button is clicked a new window pops-up and asks for username and password. The user is then logged-in. The parent page then detects using sessions that the user is logged-in and includes the text and link "Joe Bloggs: [logout]". All the public content on this website is viewed via index.php. For example, a URL to "about us" or "contact details" may be index.php?keyword=about or index.php?keyword=contact. The a user is not logged in, all links within the page work fine, but when the user IS logged in, a hyperlink from one page to another page (whose URL contains a different 'keyword') doesn't work. The browsers address bar updates to reflect the new page, but the web page never changes (even when I physically type a different 'keyword' into the address bar). Changing physical files (ie. index.php to main.php) does work. As soon as I click 'logout' which destroys the user's session, everything returns to normal. Is there something I'm missing here? Thank you for your time. Regards Tim