A common FTP data connection error of 421, 425 or 426 may be returned when running a deployment using FTP.
Most standard FTP connections use port 21, but they will use another port (or range of ports) for data transfer.
This can vary according to the FTP server that has been installed, so you will need to check with your server administrator to see what these ports are, and that they are allowed on the server's firewall for the following IP ranges:
Alternatively, it may be easier to simply whitelist the above IP ranges.
It may be required to use passive (PASV) mode for file transfer, which can help with firewall issues on your server if data ports have been restricted.