Error 12031 doing FtpConnection

I have a simple FTP transfer program that automatically logs in from a client's site and uploads error logs to our FTP site. The program is running fine at about 95% of our customer servers.

At the other 5%, though, I'm getting an error code 12031 during the put file. I've not been able to find much information on this error code, other than it means the connection was reset by the server.

Is any one able to shed more light on this error? I do know that one thing distinguishes these site from others is that you must use PASV mode during login, but I have other sites where the mode is required and they work fine.

Thanks