FTP Issues on MacOS, can't write on the disk
FTP is less used than it was due to security concerns. Do you have/ can you get access to the logs from the host? those will give more information about why the transfers are failing.

If you want a GUI you can connect as a specific user and use Screen Sharing to copy things off and on from that account as long as the host is configured to allow another computer to connect and use apple events. It can be something as simple as a guest account as long as it has a public drop box your client can drop whatever you need in there
press ⌘K to open the network connection.

If you don't care about the gui and are comfortable with the terminal:


try using ftp from the terminal -ftp will run in verbose mode giving you more information 
try using ssh to connect to the host and use ftp on the host to send you the files