IP Check policy uses lua-resty-iputils library, which doesn't support IPv6.

This library indicate to use lua-libcidr-ffi as an alternative, but it has system library dependency (libcidr) and has a low level methods. Further information could be found at lua-resty-iputils IPv6 implementation thread.

The library is used on this two methods:

local iputils = require("resty.iputils")

...

local function parse_cidrs(cidrs)
  return iputils.parse_cidrs(cidrs or {})
end

local function ip_in_range(ip, cidrs)
  return iputils.ip_in_cidrs(ip, cidrs)
end

Source: ip_check.lua

Error message at APIcast gateway:

[lua] iputils.lua:36: log_err(): Error parsing '2001:8d8:974:7c00::7a:da8c': Invalid IP, context: ngx.timer.