OS/terminal used:
$ uname -oivrs
Returns: Linux 3.16.0-4-amd64 #1 SMP Debian 3.16.39-1 (2016-12-30) unknown GNU/Linux

GNU Emacs 25.1 "ansi-term"
$ echo "$TERM"
Returns: eterm-color

Note, I have tried both xterm and urxvt as well with the same result.

Error message returned when fish_config is invoked:
[Errno 97] Address family not supported by protocol
Web config started at 'file:///home/redacted/.cache/fish/web_config-VJB6UM.html'. Hit enter to stop.
Traceback (most recent call last):
File "/usr/share/fish/tools/web_config/webconfig.py", line 1135, in <module>
ready_read = select.select([sys.stdin.fileno(), httpd.fileno()], [], [])
NameError: name 'httpd' is not defined

This bug seems to be very similar to issue #3530 which was ostensibly fixed in 2.5.0. However, for me the ability to use fish_config has regressed in the transition from fish 2.4.0 to 2.5.0; 2.4.0 being the version I was last able to use the web-based configuration.

Relevant system information:
in /etc/default/grub I have ipv6 disabled (but not completely):
GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet"

$ python -c "from __future__ import print_function; import platform, socket; print(platform.python_version(), '-', socket.has_ipv6);"
Returns: 2.7.9 - True

$ fish --version
Returns: fish, version 2.5.0

$ getent hosts localhost
Returns: ::1 localhost ip6-localhost ip6-loopback

$ sh -c 'env HOME=$(mktemp -d) fish'
Returns the same errorful output as above with only the web_config cache file name changing.