Empathy won't connect to Gtalk behind an HTTP proxy

Given:
    http_proxy=http://myproxy:3128
    https_proxy=http://myproxy:3128
    ftp_proxy=http://myproxy:3128
(set via nm-applet)

 ==>Empathy should connect to talk.google.com.

I tried UOA, empathy-account and manual editing to configure them either as "Google account", "Gtalk" and "Jabber".

    User: ................ <email address hidden>
    Password: ............ foobar
    Remember password: ... yes

    Require TLS: ......... yes
    Ignore SSL error: .... no
    Resource: ............
    Priority: ............ 20
    Server: .............. talk.google.com
    Port: ................ 443

    Use old SSL: ......... yes

 ==> Instead empathy is not connecting.

Debian's empathy is working fine with exactly the same setup.

ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: telepathy-gabble 0.16.1-2
ProcVersionSignature: Ubuntu 3.5.0-26.40-generic 3.5.7.6
Uname: Linux 3.5.0-26-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.6.1-0ubuntu10
Architecture: amd64
Date: Mon Mar 11 18:03:50 2013
InstallationDate: Installed on 2013-03-11 (0 days ago)
InstallationMedia: Ubuntu GNOME Remix 12.10 "Quantal Quetzal" - Release amd64(20121023)
MarkForUpload: True
ProcEnviron:
 LANGUAGE=fr:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: telepathy-gabble
UpgradeStatus: No upgrade log present (probably fresh install)