Your hostname is probably buggered on that particular box....
Run /scripts/ipcheck from the shell as root-
And or, as stated above
/etc/rc.d/init.d/proftpd restart
Should give you some more details.....
And, you should at least have a proftpd.conf backup in /etc/proftpd
do a ls -la | grep proftpd, inside /etc
g00d luck