421
FTP
421 Service Not Available
Service not available, closing control connection. The server is shutting down or has reached its connection limit and cannot serve this request.
Когда вы видите этот код
When the FTP server is overloaded, shutting down for maintenance, or has reached its maximum number of concurrent connections.
Как исправить
Wait a few minutes and try reconnecting. If persistent, check if the server has a connection limit and whether your IP is being rate-limited.