In /includes/functions.php we have function short_ipv6(). It will expand a compressed IPv6 (i.e. from "FADE:BAD::" to "FADE:BAD:0000:0000:0000:0000:0000:0000" in order to be able to return as many words (hexadecatets) as requested.
However, the expansion will invalidate the address if it's one with an embedded IPv4, i.e. "FADE:BAD::192.168.0.1" will become "FADE:BAD:0000:0000:0000:0000:0000:0000:192.168.0.1".
The solution is to first check for an IPv4 and replace that with two appropriate words: