Your best friend for file transfer.Fetch
Fetch displays the warning, "The data received from the server was not secure. This could mean that the server, or your connection to it, has been compromised," when you have a secure, encrypted Kerberos connection to a server and the server starts sending back unencrypted data.
If this happens, it could mean that the server, or your connection to it, has been compromised. You should disconnect and consult your network administrator or service provider.
Click the Ignore button to continue connecting, or Disconnect to not make the connection.
This warning should never appear when using SFTP or FTP with TLS/SSL to connect to a server.
Consult your network administrator or service provider if you have any question about whether the warning is significant.
You can turn off this warning by unchecking the Always check protection level on incoming data checkbox in the Security Preferences pane.