Rsync difference OS6 v OS 4 Security ?

I seem to be encountering another difference between how OS6 works compared to OS 4.

I know I can achieve my objective by using another protocol or swapping source / destination around, but that isn't really the point, so could I ask for some help please.

I am creating a third version of my data on an RN104. Having examined the 3 versions I have identified a few differences which I want to correct at an individual folder level using Rsync.

I have successfully created and run an rsync job transferring data from a local share on a Duo to a remote rsync server on a Duo. The job was created and ran first time with no issues.

I then amended the job, simply changing the destination IP address and attempted to transfer data from the first Duo to my RN104.

The job fails, for unknown reason, with the backup job log containing the following

sending incremental file list
rsync: read error: Connection reset by peer (54)
rsync error: error in rsync protocol data stream (code 12) at io.c(759) [sender=3.0.6]

I have downloaded the logs on the RN104 and system.log shows the following (IP address and destination folder edited out)

Mar 07 22:59:26 NAS_104 rsyncd[7702]: name lookup failed for 172.16.x.xx: Name or service not known
Mar 07 22:59:26 NAS_104 rsyncd[7702]: connect from UNKNOWN (172.16.x.xx)
Mar 07 22:59:26 NAS_104 rsyncd[7702]: rsync to [Destination] from UNKNOWN (172.16.0.50)
Mar 07 22:59:26 NAS_104 rsyncd[7702]: ERROR: module is read only
Mar 07 22:59:26 NAS_104 rsyncd[7702]: rsync error: syntax or usage error (code 1) at main.c(999) [Receiver=3.1.1]

I successfully tested the connection both on an anonymous basis and using the admin user / password

Any idea what is going on ? Does rsync need to be read/write, and if so how do you configure it, or is the RN104 denying access because it doesn't recognise the source NAS.