<snip> It may be an irrelevant coincidence but each FTP site that this happened with was Microsoft FTP. It never happened (ie FTP always worked fine) with other server types.
<snip> I don't know if this matters, but I had some problems recently with people downloading from external FTP servers and we found it had something to do with the FTP client and the use of either the client of external server using passive mode. I can't remember the exact details but it came down to a the ms-client/server working differently then expected...this probably won't help anyone but it might