Your posts indicate that while using rsync from the command line it works. BUT when using the Synology DSM HyperBackup app on the Synology you have an issue. That would seem to imply an issue with HyperBackup itself trying to access the My Cloud single bay v5.x device. The folks over in the Synology support forum may have additional suggestions on how to proceed.
The single bay My Cloud device is extremely limited with it’s Western Digital custom version of Linux firmware. It is entirely possible it just doesn’t support what you are attempting to do. And instead you may have to use other methods, like mapping the My Cloud to the Synology DSM using Synology’s File Manager and see if HyperBackup can access and backup to the My Cloud device.
There is the following discussion, if you haven’t seen it already, where someone claims HyperBackup worked when using a WD My Cloud EX2 Ultra.
Also, you may need to use the (ETA: My Cloud) default SSH username of “sshd” (without quotes) as well for the rsync username if you are not doing so already.
