This error corresponds to SFTP error SSH_FX_BAD_MESSAGE. It's supposed to be returned by the server if it detects a "badly formatted packet or protocol incompatibility". At the moment, we are not aware of any bug in Rebex SFTP that could make it's packets rejected (unless you use a very old version).
Please create a debug log at LogLevel.Verbose, and post the relevant part of the log that includes the "Bad message" response and several commands/responses above that. Then we can check the client packet contents to see whether there is anything wrong there.
Also, if possible, it might be useful to check the server log for any additional information on the error it reported.