Preview post not working with sub folder

I have CMSB installed in my main sites folder, and am using in on a site in a sub folder. I have got the preview function to work by updating the Preview URL to /site-subfolder/pageDetail.php

It appears to go to the right place now, however it keeps showing me this error message:
You must be logged in to use this feature!

It shows this message regardless of whether I have signed in on the site at the original URL or through the sub folder URL

Any help appreciated!