https://bugzilla.wikimedia.org/show_bug.cgi?id=61832
--- Comment #2 from Purodha Blissenbach bugzilla.wikimedia@publi.purodha.net --- Thanks for the hint. Here's the problem:
user-config.py: usernames['i18n']['i18n'] = u'Purbo_T'
pywikibot.lwp: Set-Cookie3: translatewiki_net_bw_UserName="Purbo+T"; path="/"; domain="translatewiki.net"; path_spec; expires="2014-08-22 21:18:35Z"; httponly=None; version=0
The equivalence of " ", "_", "+" inside the user name in various contexts is not properly taken unto account.
Altering user-config.py to: usernames['i18n']['i18n'] = u'Purbo T'
finds the user logged in.