This is generally because the FTP server has not correctly responded to the standard SSL/TLS closure exchange sequence. In this case, the server abruptly closes the connection without sending an acknowledgement to the client. The client can be prevented from waiting by setting DISABLE_SSL_CLOSURE:
SSLFTPClient ftp = new SSLFTPClient();
ftp.setConfigFlags(SSLFTPClient.ConfigFlags.DISABLE_SSL_CLOSURE) :
