We experienced the same symptom with the latest version 7.102 – the issue was that for some reason the symlink we used for our WP content was no longer respected by the plugin. (wpcontent was a link, we even tried a hard link) but it seems the plugin may be using the file name not full path and is then resolving the path differently. Result, it finds the ‘real’ file location, not the link.
Changed settings to the real location, and it worked – a non-optimal solution.
Posting this in case anyone has a similar issue (symptoms were failed download for Export function, and grayed Download option in Backup page – though List worked fine!).
