WD NAS to NAS backup over Internet RSync error

Hoping the collective hive-mind has struck this. Getting out of the blue daily backup failures (code 1400) after a year of trouble-free operation.
Two units - source end, a PR2100. Target end runs an EX2 Ultra.
So after a whole bunch of testing - including ensuring both ends have the latest firmware, going as far as a reset on the target end, we are getting the following out of the source end:


2024/03/11 15:23:11 [16132] cmd=/usr/bin/sshpass -e ssh -ax -o StrictHostKeyChecking=no -o HostKeyAlgorithms=+ssh-dss -c aes128-ctr -l sshd machine= user=sshd path=/mnt/HD/HD_a2/B2/WUBN21030001_Br-Print-NAS/Daily_BUP/

2024/03/11 15:23:12 [16132] rsync: connection unexpectedly closed (0 bytes received so far) [sender]

2024/03/11 15:23:12 [16132] [sender] _exit_cleanup(code=12, file=io.c, line=228): entered

2024/03/11 15:23:12 [16132] sent 0 bytes received 0 bytes total size 0

2024/03/11 15:23:12 [16132] rsync error: error in rsync protocol data stream (code 12) at io.c(228) [sender=FW_5.27.105-149-gbfc7d29a]

2024/03/11 15:23:12 [16132] [sender] _exit_cleanup(code=12, file=io.c, line=228): about to call exit(12)


We have re-created the job from scratch, rebooted, loaded latest firmware and all with the same end result.
I can create the job, browse the destination fine and even create a new directory in the job, which then is created on the target end when the job starts.

Can anyone help here or point me in the right direction?

@JeroenJ

Have you checked the sub-forums for the devices you have?

I’ve trawled the Internet for a lengthy time, including the WD forums, and so far I just haven’t found anything useful that would indicate the problem or provide any further more detailed troubleshooting.
All I can gather is that the RSync error 12 could relate to any one of about a dozen issues including missing RSync on the remote end (Test this - not the case), outdated firmware (both ends on the latest) and RSync not supporting the Blowfish encryption protocol (however boths ends are the same, and the job is set up through the GUI).
So looking for someone who can troubleshoot this a little further with me, or someone who has struck this.
This backup ran perfect for a very long time. With no changes, this now is nuked. We went as far as to (after a backup) resetting the destination unit and starting from scratch.
Ideally I don’t want to reset the source end, as there is roughly 5TB of data and is in production. We can do this, but I’d prefer some concrete accurate diagnosis to prove this is the only way.

@JeroenJ

This forum, for the WDMYCLOUD is for the single bay, 2nd generation.
WDMyCloudImage
Why did you choose to post in this forum and not the ones for your devices? Users of the same devices as you may be able to help.

No actual reason, didn’t see the “My Cloud” tag along the top beside the OS 5 NAS tag.
Have re-posted this in what I assume is the correct area.