I get a lot of the following messages in my logfile when I specify an empty value for the setting query-local-address6.
Jul 4 14:50:50 ns06 pdns-tiscom: Error trying to resolve '2a00:1478:20:7:8:772:0:1' for notifying 'saenoutdoor.nl' to server: Unable to send notify to [2a00:1478:20:7:8:772:0:1]:53:
Bad file descriptor
Fixed in commit 2772. Was not harmful, fd=-1 is never scary.