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.