After upgrading various ports on my FreeBSD system and days later a full world and kernel, a reboot showed me that unbound didn't start. The system reported that:

error: reading root hints
/usr/local/etc/unbound/named.cache 88: Empty line was returned

It turns out that from ldns 1.6.13 to 1.6.14 there is an API change that caused problems for unbound. After upgrading ldns you also need to recompile unbound to pick up on these changes. If you do not, you will run into the problem above.