I realize this is an older thread, but people may still be reading it if they encounter the same issue. I got the same error message, and it turned out that the disk was full on the server I was trying to upload to. This may not have been the case for mysat5, since someone outside the network was able to upload to the server. However, checking the available disk space (as well as file permissions) would be a good thing to start with for anyone who gets the message "Message from the Server: 'Couldn't write to remote file: SSH2_FX_FAILURE - Failure'"