Your problem is SyncToy and the underlying Framework are old, and depend upon SMB V1 to "see" a remote machine and traverse the directory tree. Most systems have depreciated SMB V1, the server-side at least. If you can turn it back
on in the remote machine it'll work again. Your NAS probably disabled it via a recent update as a [wise] security precaution.
That old framework will fallback to another method if SMBv1 fails for it; if you make an "MS Windows Compatable" SHARE on the remote it can access that SHARE correctly using the "File and Printer Sharing for Microsoft Networks"
protocol, assuming you have set the share's permissions correctly - it'll need full access basically.
Search the Web for "fix network browsing issues now that smb1 is depreciated"
