This is a reminder to myself because I've been caught by this twice:

Old versions of rsync would work fine over ssh with a target path like this:

mholmes@server.thing.ca:home/mholmes/folder/

But a particular upgrade means that you now get this error:

ERROR: module is read only
rsync error: syntax or usage error (code 1) at main.c(1034) [Receiver=3.0.9]
rsync: [sender] write error: Broken pipe (32)
rsync error: error in socket IO (code 10) at io.c(837) [sender=3.1.0]

You have to add the slash after the colon.