The plugin works fine for me, as a site admin and keymaster, both in site wide forums and in buddypress group forums, although the attachment options don't show for non admin users in buddypress forums (attachment options do show and work in site-wide forums!)
All users are allowed to upload files.
I've repaired existing users to default forum roles.
All plugins are up to date.
I've cleared the cache out from W3 Total Cache.
I've tried different browsers.
There doesn't appear to be a conflict with User Access Manager - I decativated it and had the same behaviour. I read elsewhere there might have been a problem with this.
I'm running out of ideas... any thoughts at all?