So, forgot the FTP protocol forever. The most insecure way to transfer files.
A web-based filemanager trough SSL it's another option.
Use scp protocol (Winscp software for windows, scp binary itselft on Linux)
It connects using ssh connection, so it's the most secure interface.
SFTP is using ftp trought ssh, so still remains on FTP "server" wich is not working for root.
You can enable it, but it's not secure. A user watching your LAN/Wifi traffic can get your root credentials when logging ftp with root user. That's the most important thing about "do not use root".
SCP is using ssh connection, so you can login with root perfectly, just like ssh normal session.
I used always ssh/scp with NO prob.
Regards.