I have experienced several problems accessing files on a NAS through JRiver File Explorer, including "freezes" or Explorer loosing connection to the NAS. I discovered that the problems were solved when I renamed a directory on the NAS to remove spaces from the directory name on the NAS and mapped the directory to a drive letter. I don't know if JRiver is sensitive to spaces in the directory name, or if JRiver does not handle access to NAS drives that are not mapped in Windows.
If you run into issues with your NAS, map a drive in Windows and avoid spaces in directory names. Doing so, cleared my problems with JRiver Explorer.
I am running JRiver 31.0.80