We had recently upgraded plesk to version 12 on one of the server and it showed warning that ftp service will not work unless IPV6 is enabled on the server. This happens because new version of proftpd is by default configured only listen on IPV6 ports. There is a very simple solution for this. Just go on with the Plesk upgrade and upgrade it to version 12.
You will get below error if you try to start ftp service and IPV6 is disabled on your server
server xinetd[503]: socket creation failed (Address family not supported by protocol (errno = 97)). service = ftp server xinetd[503]: Service ftp failed to start and is deactivated.
Resolution
Edit the file /etc/xinetd.d/ftp_psa
service ftp { flags = IPv6 disable = no socket_type = stream
replace “flags = IPv6” with “flags = IPv4”
service ftp { flags = IPv4 disable = no socket_type = stream
Your solution did not work. I am using Filezilla with Plesk 12.
What error are you getting in the server logs?