Can not set ipv6 address of vlan interface

pox

I have a vlan200 interface, and I wanted to set a static ipv6 address of
fdf5:4443:aabb:200::1/64

but after hitting apply in pfsense, i get a "The changes have been applied successfully." message, but in reality it fails:

php-fpm[92471]: /interfaces.php: The command '/sbin/ifconfig 'igb1_vlan200' inet6 fdf5:4d23:ab9b:200::1 prefixlen '64'' returned exit code '1', the output was 'ifconfig: ioctl (SIOCAIFADDR): File exists'

unbound stops working too, because

unbound: [77852:0] error: cannot parse netblock: '/'
unbound: [77852:0] error: cannot parse netblock: '/'
unbound: [77852:0] error: cannot parse access control: / allow
unbound: [77852:0] error: cannot parse access control: / allow

But if instead of setting the ipv6 address of the interface to
fdf5:4443:aabb:200::1/64
I set it to
fdf5:4443:aabb:20::1/64

everything works as expected.

Am I doing something wrong here? Can someone please help me understand what's going on?