Role root directory won't save - all users without administer webfm get No trees found

This is an odd problem, it's also the first time I've used Drupal so maybe it's something wrong elsewhere.

I've installed WebFm and set the Web FM root directory to /sites/default/files/downloads, this all works fine with the admin user. I can upload and download files fine.
My aim is for authenticated users to only download files so edited the authenticated user role to only have 'access webfm'. When I log in with anyone other than admin I now get a webfm menu item and clicking on this shows a blank webfm page with the error 'No trees found'
This is understandable as the user doesn;t have a root directory set, so under 'settings for authenticated user' i create a folder called 'public' and save the config. This creates the folder called /sites/default/files/downloads/public with the correct permissions but doesn't resolve the problem, I still get 'No trees found'.
If I exit from the webfm configuration then go back in the 'role root directory' is blank again and resetting it makes no difference.

I've set up a seperate role called 'customer' and set the role root directory to /sites/default/files/downloads/customer yet I still get exactly the same problem, the role root directory value disapears.

If i give the other user admin or upload rights then try to upload a file I get 'Invalid upload path' so it looks as though the role root path isn't being set.

This is running on LAMP, SLES 10 I think.