I suggest you first check the server-side logs.
The "Force IP" cannot make difference at this stage.

Hi

We have a ftp site set up in iis with a self signed certificate. We can connect to it ok internall and externally but one customer is having problems trying to connect to it using SCP.

The log file they get is:
. 2012-09-04 16:04:33.933 --------------------------------------------------------------------------
. 2012-09-04 16:04:33.933 WinSCP Version 4.3.9 (Build 1817) (OS 5.1.2600 Service Pack 3)
. 2012-09-04 16:04:33.933 Configuration: HKEY_CURRENT_USER\Software\Martin Prikryl\WinSCP 2\
. 2012-09-04 16:04:33.933 Local account: SERES\gustavo
. 2012-09-04 16:04:33.933 Login time: martes, 04 de septiembre de 2012 16:04:33
. 2012-09-04 16:04:33.933 --------------------------------------------------------------------------
. 2012-09-04 16:04:33.933 Session name: ALCUMUS-ftps (Modified stored session)
. 2012-09-04 16:04:33.933 Host name: 89.238.166.189 (Port: 21)
. 2012-09-04 16:04:33.933 User name: Ferrovial (Password: Yes, Key file: No)
. 2012-09-04 16:04:33.933 Tunnel: No
. 2012-09-04 16:04:33.933 Transfer Protocol: FTP
. 2012-09-04 16:04:33.933 Ping type: C, Ping interval: 30 sec; Timeout: 15 sec
. 2012-09-04 16:04:33.933 Proxy: none
. 2012-09-04 16:04:33.933 FTP: FTPS: Explicit TLS; Passive: Yes [Force IP: +]
. 2012-09-04 16:04:33.933 Local directory: default, Remote directory: home, Update: No, Cache: Yes
. 2012-09-04 16:04:33.933 Cache directory changes: Yes, Permanent: Yes
. 2012-09-04 16:04:33.933 DST mode: 1
. 2012-09-04 16:04:33.933 --------------------------------------------------------------------------
. 2012-09-04 16:04:34.027 Connecting to 89.238.166.189 ...
. 2012-09-04 16:04:34.105 Connected with 89.238.166.189, negotiating SSL connection...
< 2012-09-04 16:04:34.199 220 Microsoft FTP Service
> 2012-09-04 16:04:34.199 AUTH TLS
< 2012-09-04 16:04:34.277 234 AUTH command ok. Expecting TLS Negotiation.
. 2012-09-04 16:04:34.402 Disconnected from server
. 2012-09-04 16:04:34.402 Connection failed.
* 2012-09-04 16:04:34.402 (ESshFatal) Connection failed.
* 2012-09-04 16:04:34.402 Disconnected from server
* 2012-09-04 16:04:34.402 Connection failed.
* 2012-09-04 16:04:34.402 AUTH command ok. Expecting TLS Negotiation.

The only difference I can see between their log files and mine is that they get:
FTP: FTPS: Explicit TLS; Passive: Yes [Force IP: +]

Whereas I and everyone else i got to test it gets:
FTP: FTPS: Explicit TLS; Passive: Yes [Force IP: A]

Can anyone help me with this?

Thanks
Ben