Oops, didn't finish my initial comment.
IPv6 users typically can get more than a /64, so CheckUser needs to be able to check more than that.
What range would make sense do you think? (Btw. I have fixed the message, it used to say /96-/128).
/48, since that's usually what's usually considered the equivalent of a /16 in IPv4.
Change 203679 had a related patch set uploaded (by saper):
Allow CheckUser for IPv6 ranges up to /32