IPv6 support was fixed in httpclient 2.4.0 and newer.

https://trello.com/c/NoHDPqHH

See: https://github.com/nahi/httpclient/blob/master/CHANGELOG.md

Related changelog entries:

2.4.0:
* Remove [] from given address for IPv6 compat. #176

2.3.1:
* #122 Support IPv6 address in URI
Let URI build the URI since hostname= auto-wraps an ipv6 address in [].

https://trello.com/c/NoHDPqHH