he repository 'file:/var/cache/openmediavault/archives Release' does not have a Release file.
Try (as root):
echo "deb [trusted=yes] file:/var/cache/openmediavault/archives /" > /etc/apt/sources.list.d/openmediavault-local.list
apt-get update
Then post the output of:
apt-cache policy proftpd-basic