Adding music files, that are stored on the NAS.
There are 2 options of adding storage locations in Roon: By adding local files and by adding a network share. You might be used to adding files from a NAS by network share (because it is a network device, right?), but when you’re core is installed on a NAS is is wrong to point your core to your music files over network!
As long as Roon Server and the music files are stored on the same device you should always add them as a local folder.
Only if you have your core on a different device in the network, you should use the option to add music files by a network share.