We helped rebuild a backup VM which receives data over rsync today; the first rsync attempt failed with the vague error message:
rsync error: error in rsync protocol data stream (code 12) at io.c(600) [sender=3.0.6]
The problem ended up being that the rsync package was not installed. A simple:
yum install rsync -y
…fixed the problem and shortly the backups were back to normal.