I have had the same problem. It seems to be a bug in the proftpd init script that causes the ftp server to fail to start. See https://bugs.launchpad.net/ubuntu/+source/proftpd-dfsg/+bug/1246245
A workaround is to change the config to include
copytruncate Instead of moving the old file and creating a new one, this will copy the contents of the file and then truncate the old one - meaning that you don't need to restart the daemon.
