maybe bad Interface
Please post your output of 'ifconfig' for your existing interfaces.
And ist the ftp really listen on the "right" interface ? Seems to be only
on your loopback. Try 'netstat -anp | grep :21' so see this.
Maybe could could try the Option "Defaultserver on" ist your proftpd.conf
Sometimes, wenn this option is missing, the daemon couldn't bind on
all/external interface. Your .conf seems to be ok, IMHO. After making
this change, you have to restart the daemon.