The internet failed to tell me:
This error can result from not having one of the directories present. Yes, I know that rsync creates lots of directories very nicely. Not all of them.
rsync someDirectory someUser@18.104.22.168:/home/someUser/system/code
Gave the the data stream error until I created the directory 'system'.
I can't imagine why. The only thing that is distinctive about 'system' is that it is also ~/system, ie, at the top of the user's directory.