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My client has sent the following report.

FTP : AUTH: security mechanism 'SSL' not supported (504).

We can not get more informations about this error because no other logs can be obtained from client.
Please let me know if you know the cause of this error and how to fix it.

Thanks you.

Applies to: Rebex FTP/SSL

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When an FTP client such as Rebex FTP/SSL intends to establish a secure connection using explicit TLS/SSL, it connects to the server without using any encryption and issues AUTH TLS or AUTH SSL command. When both of these commands fail, it means it's not possible to establish an explicit TLS/SSL connection, and an exception is thrown based on the error message received from the server - this is the error you see.

This means that the client might be trying to connect to an FTP server that does not support TLS/SSL (or that it only supports implicit TLS/SSL.