We recently switched our server to require TLS for every connection. This is working flawlessly so far, but one of our clients is having problems.

Some facts:

• Server uses Pure-FTPD
• Server has a passive port range configured
• Server has no firewall limitations regarding the FTP
• Client uses WS FTP
• Client is behind a router
• Client connects to the same IP as every other, using PASSIVE mode
• All other clients have no trouble connecting

Because of the TLS requirement, connecting using ACTIVE mode is almost not possible, but PASSIVE is working fine for everyone except this specific client. It seems that he is able to connect, but once a LIST command is performed, things go wrong.

Log:

Finding Host <clienthost> ...
Connecting to <serverip:21>
Connected to <serverip:21> in 0.020000 seconds, Waiting for Server Response
Initializing SSL Session ...
220---------- Welcome to Pure-FTPd [privsep] [TLS] ----------
220-You are user number 5 of 50 allowed.
220-Local time is now 22:14. Server port: 21.
220-This is a private system - No anonymous login
220-IPv6 connections are also welcome on this server.
220 You will be disconnected after 15 minutes of inactivity.
AUTH TLS
234 AUTH TLS OK.
SSL session NOT set for reuse
SSL Session Started.
Host type (1): Automatic Detect
USER <user>
331 User <user> OK. Password required
PASS (hidden)
230-User <user> has group access to:  <user>   
230 OK. Current restricted directory is /
SYST
215 UNIX Type: L8
Host type (2): Unix (Standard)
PBSZ 0
200 PBSZ=0
PROT P
200 Data protection level set to "private"
PWD
257 "/" is your current location
CWD /public_html
250 OK. Current directory is /public_html
PWD257 "/public_html" is your current location
TYPE A
200 TYPE is now ASCII
PASV
227 Entering Passive Mode (<serverip>,132,100)
connecting data channel to <serverip>:132,100(33892)
Substituting connection address <serverip> for private address <serverip> from PASV
Using external address <customer ext. ip> instead of local address <customer int. ip> for PORT command
PORT 82,161,56,225,195,181
200 PORT command successful
LIST
Error reading response from server.
It appears that the connection is dead.  Attempting reconnect...

Any help is appreciated.