I had this exact problem. I added run_dhclient="false" to /etc/default/ddclient
since I run ddclient in daemon mode it should not run from dhcpclient. you have to set it to something else the /etc/dhcp/dhclient-exit-hooks.d/ddclient script fails. or you could fix the problem in /etc/dhcp/dhclient-exit-hooks.d/ddclient by changing [ $run_dhclient = "true" ] || exit 0 to [ "$run_dhclient" = "true" ] || exit 0
