Fatal error: Received unexpected header from the server:
 expected "Unison 2.48\n" but received "Unison 2.40\n\000\000\000\000\017", 
which differs at "Unison 2.40".
This can happen because you have different versions of Unison
installed on the client and server machines, or because
your connection is failing and somebody is printing an error
message, or because your remote login shell is printing
something itself before starting Unison.