Link to pdf file in s2members-files shows folder

I have links to multiple files in subfolders under wp-content/plugins/s2member-files. These links have always worked fine, but now when a logged in user clicks on the link a folder listing shows in the browser instead. Not sure what has changed or when it stopped working (it could be any time in the last three months) and I haven’t a clue where to start looking, but I have tried re-installing the framework and the pro plugin. I have also tried replacing the .htaccess in the root with one from an old backup. Normal links to PDF files work fine too - any help appreciated