Ahhh – it appears this could be a permission issue. As Firefly runs as a service, the run-as account needs to be set to match a user with permission to access the NAS box.
This is not entirly apparent as the person trying to install and set up the server is inevitably logged in to the console as a user who DOES have access to the data; suggest the installation notes are ammended with a section reminding the installer that the server runs as a service and that the service’s run-as parameters may need to be set.
(Same applies to SlimServ of course!)