DHCPv6 on WAN not working

[reinderien]

(This is mirrored at https://www.reddit.com/r/PFSENSE/comments/86hjs2/default_dhcp6_broken/)

I am running the latest pfSense, 2.4.2-RELEASE-p1 (amd64). My WAN interface is configured to use DHCP6, but fails, putting this in the logs:

Mar 22 22:28:09 dhcp6c  26391   failed to open /usr/local/etc/dhcp6cctlkey: No such file or directory
Mar 22 22:28:09 dhcp6c  26391   failed initialize control message authentication
Mar 22 22:28:09 dhcp6c  26391   skip opening control port
Mar 22 22:28:09 dhcp6c  26391   /var/etc/dhcp6c_wan.conf:3 IA_PD (0) is not defined
Mar 22 22:28:09 dhcp6c  26391   failed to parse configuration file

The LAN is configured with a static, local IPv6 address but this shouldn't affect the WAN. Even though the WAN DHCP6 is not working, clients can still get to the IPv6 internet via the WAN's link-local address. I haven't changed any of the advanced settings in the DHCP config on the interface, and IPv6 is enabled globally. I can confirm that other clients on the same network as the WAN do get v6 leases.

What could be the problem here?