You can use FFP in two ways. You can try to change the settings of the firmware ftp server, or you can use the ftp server provided with FFP
Personally I'd choose the last option. Trying to change the settings of a firmware configuration file can by cumbersome, as the firmware can change these files dynamically.
I don't know which file you need to change. On my fairly outdated firmware v2.10 there is a file /etc/vsftpd.conf, but strangely enough the available ftp server is pure-ftpd (found in /usr/local/sbin). So I have no idea which ftp server is running on newer firmware. I suppose you can find out by running
ps -A | grep ftp
or just have a look in /etc/original/, where the firmware configuration files are.
(I suppose you know that you can get rid of this error by subdividing the data in several subdirectories?)