The remote host is vulnerable to a Denial of Service (DoS) attack.
The remote host appears to be using SlimFTPd, a free, small standards-compliant FTP server for Windows. The installed version of SlimFTPd on the remote host suffers from a denial of service vulnerability. By sending 'user' and 'pass' commands that are each 40 bytes long, an attacker will crash the service after a short period of time.
No solution is known at this time.