[update, solved] It was apparmor, which was lying about being inactive. Ubuntu's default profile denies bind write access to its config directory. Needed to add /etc/bind/dnskeys/** rw, reload apparmor, and it's all good.

Trying to switch my internal domain from auto-dnssec maintain to dnssec-policy default. Zone is signed but not secure and logs are full of

zone_rekey:dns_dnssec_keymgr failed: error occurred writing key to disk

key-directory is /etc/bind/dnskeys, owned bind:bind, and named runs as bind

I've set every directory I could think of to 777: /etc/bind, /etc/bind/dnskeys, /var/lib/bind, /var/cache/bind, /var/log/bind. I disabled apparmor, in case it was blocking.

A signed zone file appears, but I can't dig any DNSKEYs or RRSIGs. named-checkzone says there's nsec records in the signed file, so something is happening, but I'm guessing it all stops when keymgr fails to write the key.

I tried manually generating a key and sticking it in dnskeys, but this doesn't appear to be used.