Short: It looks like the FTP command "LIST /" does not have the expected
effect on this server.
Reproduce: ftp.microsoft.com   anonymous login.

-----------------------------------------------------------------
FTP Log Session: Wed Jul 18 17:58:40 GMT+02:00 2001
-----------------------------------------------------------------
220 ******* Microsoft FTP Service (Version 5.0).
USER ******
331 Password required for teikom.
PASS *****
230-****** testsite
230 User ****** logged in.
SYST
215 Windows_NT version 5.0
TYPE I
200 Type set to I.
PORT 192,168,1,32,16,171
200 PORT command successful.
LIST /
150 Opening BINARY mode data connection for /bin/ls.
226 Transfer complete.
PORT 192,168,1,32,16,172
200 PORT command successful.
LIST /
150 Opening BINARY mode data connection for /bin/ls.
226 Transfer complete.
PORT 192,168,1,32,16,179
200 PORT command successful.
LIST /
150 Opening BINARY mode data connection for /bin/ls.
226 Transfer complete.
PORT 192,168,1,32,16,196
200 PORT command successful.
LIST /
150 Opening BINARY mode data connection for /bin/ls.
226 Transfer complete.
PORT 192,168,1,32,16,202
200 PORT command successful.
...
...
Just continues like above...

Manual command line session:

C:\rrib\netbeans\ftpcache>ftp ****************
Connected to *********.
220 ********* Microsoft FTP Service (Version 5.0).
User (************:(none)): ******
331 Password required for ******.
Password:
230-****** testsite
230 User ****** logged in.
ftp> LIST /
Invalid command.
ftp> LIST /
Invalid command.
ftp> ls -l
200 PORT command successful.
150 Opening ASCII mode data connection for /bin/ls.
...
06-11-01  11:16AM                 1865 index.html
...
226 Transfer complete.
ftp: 4624 bytes received in 0,37Seconds 12,50Kbytes/sec.
ftp> ls
200 PORT command successful.
150 Opening ASCII mode data connection for file list.
...
index.html
...
226 Transfer complete.
ftp: 1309 bytes received in 0,23Seconds 5,69Kbytes/sec.