Yes the user is an admin on the NAS with full permissions. It has been working for the past couple of years. If I only backup the config it works fine, but if I add in some extra bits, like voicemail, or chat messages, etc. The initial 11Meg file is created and starts transferring, then the other components are being added (I can see the file growing in /tmp as the transfer is taking place. The ProFTP server log shows the error. 3CX comes back saying job done, but the file is never listed and looking at it on the NAS it is a 0 byte file.
This is what the proftp log looks like when it fails.
Tue Sep 26 07:12:56 2017 0 192.168.50.15 653048 /var/ftp/backup/current/zultys/3cx/3CXScheduledBackup.zip b _ o r admin ftp 0 * i
2017-09-26 07:12:56,686 nas proftpd[31583] 127.0.0.1 (192.168.50.15[192.168.50.15]): notice: user admin: aborting transfer: Data connection closed
The first line is the Proftp server writing the file. If I tail -f the log, I see it begin the transfer before the /tmp file on the 3CX is finished growing.