If you use Firefox, check that you haven't turned of the HTTP Referer header, that could likely cause problems like this.
To do this, type in
about:config in the address field. This should display the list of Firefox settings in the browser window. In the
Filter input box at the top, type in
network.http.sendRefererHeader. It should appear in the list below. Check that the value is set to
2. If it's not, set it to 2.
