Have you tried to enable SSH on the EX4100 and check if FTP service is running on the unit ?
root@WDMyCloudEX4100 root # ps -aef | grep ftp
root 7248 1 0 2017 ? 00:00:00 pure-ftpd (SERVER)
root 9212 9180 0 20:19 pts/0 00:00:00 grep ftp
If this process is not running that will be an issue. Also have you tried restarting the EX4100 to see if FTP becomes available after a reboot ?
The EX4100 uses ext4 as the backing filesystem for the RAID volume by the looks and this filesystem, can be configured to have space reserved that only the “root” user can write to when it is getting close to full. If you are able to, maybe see if you can delete some stuff that is not required to free up some space, and then test again if the FTP service works. Try and reboot again once the space is cleared as well and test.
Beyond that, they may be indications in the logs on the EX4100 to indicate why the FTP service is not running, but these would need to be access via SSH as well and are cleared each time the unit is rebooted.