It sounds as if you have hit a bug in the windows implementation of the CIFS/SMB protocol and/or the way win 10 sees free disk space on non windows formatted disks.
Something you could try.
Activate the FTP server on the DNS and attach it with read/write to the user you have for CIFS/SMB.
Download a copy of FileZilla (an FTP client), install it and then connect to the DNS using the IP address, username and password. Then use that to transfer the files from your computer to the NAS.
If FTP works use that until the next beta update of win 10 is applied to your computers.