What could be stoping async ftp?

I have a client that went from the following configuration:

old config:
workstation -> router -> adsl modem -> internet -> foreign host

to

new config:
workstation -> server -> adsl modem -> internet -> foreign host


In each case the workstation is running mandrake 9.1 with win4lin
running windows 98.

The server in the new config is running mandrake 9.0 pro suite. It is
using Bastille as a firewall(don't ask me why, it's a preference thing
of the client).

Here is the problem:

Under the old config, the client was able to launch a special program in
windows in win4lin. This program would create a special report and ftp
the report to a FTP server outside their network. No problem.

Under the new config, the client repeats the same steps but the FTP
server returns an async error message and drops the connection (sorry, I
don't have the exact error in front of me).

It appears the new config is some how blocking or disabling the async
ftp from happening.

Anyone know where I can find more info on async ftp (all the googling
I've done just points to windows stuff) or better yet, what the cause of
the problem is and the solution?


Thanks

David