Hi Tim,
There is a way to get around of this so you won't get this nasty error. However, it's not the best idea to do, but if you're not using ftp at all, it might be worth it.
The logrotation process is done by a cron job in /etc/cron/cron.daily From there it starts the program "logrotate" which has it's config file in /etc/logrotate.conf. In there you'll find that more config options are comming from /etc/logrotate.d Now, to make sure you don't get these errors anymore, just open up ftpd in an editor and comment everything out and you're done!
Please note that when you'll start using the ftp-service, eventually your logs may start growing. In a worst case scenario, this could fill your disk, preventing your system from working! So, the choice is yours: getting the e-mail or live on the risky side!
bibi,
arthur