sipcalc does not support ipv6 zone index identifiers. Since /etc/util.subr uses sipcalc this makes it fail in calculating a subnet if using an ipv6 address with a zone identifier. This has the practical effect of being unable to use link local addresses for NFS exports.
I've attached a patch to work around this issue by stripping the zone identifier from the ipv6 address before sending to sipcalc. Obviously the proper way to fix it would be to fix sipcalc, but I don't know C. Hopefully this patch can be incorporated in the interim.
Obviously related is the fact that the web interface doesn't allow ipv6 addresses with zone identifiers, and doesn't allow a selection of a mask greater than 32, so the only way to do this in the first place is to manually edit conf.xml to add the IP.