There may be several reasons for a connection problem on an SFTP server, but it is not always easy to understand the message that explains the reason.So here is a table of the different errors that can be encountered with a description.

Definition Value Description
ERROR_SSH_INVALID_IDENTIFICATION_STRING 1 (0x0001) Invalid identification string of SSH-protocol
ERROR_SSH_INVALID_VERSION 2 (0x0002) Invalid or unsupported version
ERROR_SSH_INVALID_MESSAGE_CODE 3 (0x0003) Unsupported message code
ERROR_SSH_INVALID_CRC 4 (0x0004) Message CRC is invalid
ERROR_SSH_INVALID_PACKET_TYPE 5 (0x0005) Invalid (unknown) packet type
ERROR_SSH_INVALID_PACKET 6 (0x0006) Packet composed incorrectly
ERROR_SSH_UNSUPPORTED_CIPHER 7 (0x0007) There is no cipher supported by both: client and server
ERROR_SSH_UNSUPPORTED_AUTH_TYPE 8 (0x0008) Authentication type is unsupported
ERROR_SSH_INVALID_RSA_CHALLENGE 9 (0x0009) The wrong signature during public key-authentication
ERROR_SSH_AUTHENTICATION_FAILED 10 (0x000A) Authentication failed. There could be wrong password or something else
ERROR_SSH_INVALID_PACKET_SIZE 11 (0x000B) The packet is too large
ERROR_SSH_HOST_NOT_ALLOWED_TO_CONNECT 101 (0x0065) Connection was rejected by remote host
ERROR_SSH_PROTOCOL_ERROR 102 (0x0066) Another protocol error
ERROR_SSH_KEY_EXCHANGE_FAILED 103 (0x0067) Key exchange failed
ERROR_SSH_INVALID_MAC 105 (0x0069) Received packet has invalid MAC
ERROR_SSH_COMPRESSION_ERROR 106 (0x006A) Compression or decompression error
ERROR_SSH_SERVICE_NOT_AVAILABLE 107 (0x006B) Service (sftp, shell, etc.) is not available
ERROR_SSH_PROTOCOL_VERSION_NOT_SUPPORTED 108 (0x006C) Version is not supported
ERROR_SSH_HOST_KEY_NOT_VERIFIABLE 109 (0x006D) Server key can not be verified
ERROR_SSH_CONNECTION_LOST 110 (0x006E) Connection was lost by some reason
ERROR_SSH_APPLICATION_CLOSED 111 (0x006F) User on the other side of connection closed application that led to disconnection
ERROR_SSH_TOO_MANY_CONNECTIONS 112 (0x0070) The server is overladen
ERROR_SSH_AUTH_CANCELLED_BY_USER 113 (0x0071) User tired of invalid password entering
ERROR_SSH_NO_MORE_AUTH_METHODS_AVAILABLE 114 (0x0072) There are no more methods for user authentication
ERROR_SSH_ILLEGAL_USERNAME 115 (0x0073) There is no user with specified username on the server
ERROR_SSH_INTERNAL_ERROR 200 (0x00C8) Internal error of implementation
ERROR_SSH_NOT_CONNECTED 222 (0x00DE) There is no connection but user tries to send data
ERROR_SSH_CONNECTION_CANCELLED_BY_USER 501 (0x01F5) The connection was cancelled by user
ERROR_SSH_FORWARD_DISALLOWED 502 (0x01F6) SSH forward disallowed
ERROR_SSH_ONKEYVALIDATE_NOT_ASSIGNED 503 (0x01F7) The event handler for OnKeyValidate event, has not been specified by the application
ERROR_SSH_GSSKEX_SERVER_ERROR_MESSAGE 601 (0x0259) GSS KEX server error.
ERROR_SSH_GSSAPI_SERVER_ERROR_MESSAGE 603 (0x025B) GSS API server error.
ERROR_SSH_TCP_CONNECTION_FAILED 24577 (0x6001) TCP connection failed. This error code defined in SBSSHForwarding unit.
ERROR_SSH_TCP_BIND_FAILED 24578 (0x6002) TCP bind failed. This error code defined in SBSSHForwarding unit.