Hopefully this will save someone else some time & frustration.
I am deploying a new-smelling DNS345 to do backups of several remote servers. All have current rsync.conf files that have worked swimmingly for quite a while.
The DNS345 failed out with an 'RSync Test failed' while trying to configure the backup. The server log at the other end said 'Password failure'.
It turns out that the DNS345 doesn't/can't send a long password. The existing password was 17 characters. One shortened down to 8, all worked well.