This might not apply to you, but if it is a Linux FTP server:
This may help for Linux FTP server.
So, Linux FTP servers unlike IIS don't have common FTP root directory.
Instead, when you log on to FTP server under some user's credentials,
this user's root directory is used. So FTP directory hierarchy starts
from /root/ for root user and from /home/username for others.
So, if you need to query a file not relative to user account home
directory, but relative to file system root, add an extra / after
server name. Resulting URL will look like:
ftp://servername.net//var/lalala
Instead of:
ftp://xxx.com/public_html
You would need a second slash after the server name in addition to the full file name:
ftp://xxx.com//public_html/adress.txt
I ran into this same issue and it fixed it for me.
Source:
Can't connect to FTP: (553) File name not allowed