Could you post your user-config.py?

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.

It's just the _ in the user name -- we determine user names like this:

if not self.nocapitalize:                                                                                                                                                                                          if user:
         user = user[0].upper() + user[1:]                                                                                                                                                                          if sysop:
         sysop = sysop[0].upper() + sysop[1:]

(site.py)

and compare that to what the API returns (which is a capitalized-or-not name with spaces, not underscores).

The better solution would be to either
a) normalize the username (with a Page object -- which I guess is OK because Page objects are used more often in Site)
or
b) comparing the usernames with site.sametitle (which currently does not take underscores into account, but should)

Marking as 'easy' for anyone willing to pick up option a)

Change 117689 had a related patch set uploaded by Purodha:
Bug: 61832 - fixed.

https://gerrit.wikimedia.org/r/117689

Change 119882 had a related patch set uploaded by Tim Landscheidt:
become: Add --help option

https://gerrit.wikimedia.org/r/119882

Sorry, comment #5 was a typo of mine.

https://gerrit.wikimedia.org/r/117689 merged.

Change 150872 had a related patch set uploaded by John Vandenberg:
WIP: Introduce static method Link.normalize(title)

https://gerrit.wikimedia.org/r/150872