The remote host is vulnerable to a Denial of Service (DoS) attack.
The remote host is running SurgeFTP, an FTP server for Microsoft and Unix platforms. This version of SurgeFTP is vulnerable to a content-parsing flaw via the LEAK command. Calling the LEAK command twice consecutively causes a file IO bug that causes the server to either stop responding or stop file transfers.
Upgrade to version 2.2m2 or higher.