trying to backup from Ultra 6+ to new 316

I have not configured rsync user names on my pro nor my RN102. As far as security goes, I don't think they add any real security for backup unless you are running rsync over ssh. (why bother if all the files are backed up in the clear?) I also see no compelling reason to avoid using user accounts, the documentation just says it doesn't matter, not that it increases security. Just my opinions though.

There are ten basic combinations here
One should fail:
-no user credentials specified in the backup job, but credentials configured for rsync at the remote share
(there are other combinations that ought to fail that I am ignoring).

The other 9 should work:
-no user credentials specified in the backup job, and none configured for rsync at the remote share
-user credentials specified in the backup job that exist only on the remote machine, but no rsync users configured at the remote share
-user credentials specified in the backup job that exist only on the remote machine, and only that user configured for rsync at the remote share
-user credentials specified in the backup job that exist only on the local machine, but no rsync users configured at the remote share
-user credentials specified in the backup job that exist only on the local machine, and only that user configured for rsync at the remote share
-user credentials specified in the backup job that exist on neither machine, and only that user configured for rsync at the remote share
-user credentials specified in the backup job that exist on neither machine, but no rsync users configured at the remote share
-user credentials specified in the backup job that exist on both machines, and only that user configured for rsync at the remote share
-user credentials specified in the backup job that exist on both machines, but no rsync users configured at the remote share

Since this is a bug, behavior might depend on which machine is running the backup job - so really there are 20 basic combinations total.

In my own backup jobs, the RN102 uses the pro admin credentials in the rsync backup job, even though there is no rsync user configured on the pro for that share. The RN102 admin credentials are also the same as the pro's, so that would be the last combination above.

Maybe try some (or all) of the above combinations on a small test share, and see which work. It should give you a better handle on your options (and perhaps something specific to put into your support request).

BTW, what firmware are you running on the RN316?