unable to do FTP to Windows FTP server from F5 device

Assuming that 192.168.12.2 is a pool member running an FTP server, and not a virtual server that load balances to a pool, then the error message is pretty clear in that the remote end (the FTP server) closed the connection. I suggest you check the logs on that device to determine why it did that. If there are any firewalls or NAT devices between the BigIP and the FTP server, then also check on them in case something sent a RST.

If 192.168.12.2 is actually a virtual server on the LTM, then verify whether the ftp works if you go directly to the pool member.