I see this in my rsync man page:
rsync occasionally produces error messages that may seem a little cryptic. The one that seems to cause the
most confusion is "protocol version mismatch -- is your shell clean?".
This message is usually caused by your startup scripts or remote shell facility producing unwanted garbage on
the stream that rsync is using for its transport. The way to diagnose this problem is to run your remote
shell like this:
ssh remotehost /bin/true > out.dat
then look at out.dat. If everything is working correctly then out.dat should be a zero length file. If you
are getting the above error from rsync then you will probably find that out.dat contains some text or data.
Look at the contents and try to work out what is producing it. The most common cause is incorrectly configured shell startup scripts (such as .cshrc or .profile) that contain output statements for non-interactive