When creating an HTTPClientSession the Host:port are provided. For IPv6 hosts, the brackets are stripped when sending the host field. So an address like:
[fc00:121:121:64::66]:8081 incorrectly appears in the host field as fc00:121:121:64::66:8081.
There is a workaround to explicitly set host on the Request to override the field with the correct value.
fixed in develop and 1.4.6p2