hotspotlogin.cgi problem

Hi, I have set up a FreeRadius server which I am trying to access from Chilli on a Linksys AP running DD-WRT and Chilli. Radtest shows that the Radius server is running and Chilli is directing users to /cgi- bin/hotspotlogin.cgi. However this produces an internal server error from Apache. If I send users to /cgi-bin/test.cgi the test page loads with no errors. I have no idea where to start looking for a solution. Any advice or pointers to where I've gone wrong would be very much appreciated. Regards, Sean Bracken