--stats in 3.1.0 when connecting to an rsync 3.0.5
/usr/local/s3rsync/prod-ssh-rsync3: invalid rsync-command syntax or options
You didn't provide any info on what was actually being run, but the cited error makes it look like an rrsync script is being used. Make sure it is updated to the latest version since rsync can now send --stats to the server.
I just checked to see if rrsync was fully up-to-date, and found a few more new options that were missing from the just-released rrsync script (e.g. --preallocate). If you want the very-very latest, get it from here: