SSH code of WinSCP is based on PuTTY. So if you are getting some error message while using SFTP or SCP protocols (particularly while connecting), you may check also the common error messages in PuTTY documentation.

• Server sent disconnect message type 2 (protocol error): “Too many authentication failures for root”
• Unable to use this private key file, Couldn’t load private key, Key is of wrong type
• Unexpected directory listing line ‘…’
• Received too large (… B) SFTP packet. Max supported packet size is 102400 B
• Command failed with return code 127 (or 255)
• WinSCP was not able to determine application that was started to open the file. WinSCP cannot watch for changes in the file and thus it will not upload the changed file back
• Error skipping startup message. Your shell is probably incompatible with the application (BASH is recommended)
• Server returned empty listing for directory
• Timeout waiting for external console to complete the command
• Error listing directory
• Cannot initialize SFTP protocol. Is the host running a SFTP server?
• The server cannot open connection in active mode. If you are behind a NAT router, you may need to specify an external IP address. Alternatively, consider switching to passive mode.
• The requested name is valid and was found in the database, but it does not have the correct associated data being resolved for
• Transfer was successfully finished, but temporary transfer file … could not be renamed to target file name …
• Upload of file .. was successful, but error occurred while setting the permissions and/or timestamp. If the problem persists, turn off setting permissions or preserving timestamp. Alternatively you can turn on ‘Ignore permission errors’ option.
• The supplied message is incomplete. The signature was not verified.
• WinSCP process terminated with exit code 3
• Timeout waiting for WinSCP to respond
• Cannot initialize external console
• Could not load file or assembly ‘file:///…\WinSCPnet.dll’ or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515)
• Could not load file or assembly ‘file:///…\WinSCPnet.dll’ or one of its dependencies. The system cannot find the file specified.
• Could not load file or assembly ‘file:///…\WinSCPnet.dll’ or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.
• Exception has been thrown by the target of an invocation
• SessionOptions.Protocol is Protocol.Sftp or Protocol.Scp, but SessionOptions.SshHostKeyFingerprint is not set
• Selecting files using a path ending with slash is ambiguous. Remove the slash to select the folder. Append * mask to select all files in the folder.
• Sending special code: 12/15
• No mapping for the Unicode character exists in the target multi-byte code page – The file must be in UTF-8 or UTF-16 encoding.
• Invalid access to memory

SSH code of WinSCP is based on PuTTY. So if you are getting some error message while using SFTP or SCP protocols (particularly while connecting), you may check also the common error messages in PuTTY documentation.