BugTrackerAuthenticationError: 401 "Unauthorized"

As seen on OOPS-1866CCW1292, an unauthorized error happens when trying to access gobby-bugs bugtracker (trac):

    Traceback (most recent call last):
  Module lp.bugs.scripts.checkwatches.core, line 245, in updateBugTracker
    self._updateBugTracker(bug_tracker, batch_size)
  Module lp.bugs.scripts.checkwatches.core, line 424, in _updateBugTracker
    remotesystem, bug_watch_batch, batch_size=batch_size)
  Module lp.bugs.scripts.checkwatches.base, line 266, in wrapper
    return func(self, *args, **kwargs)
  Module lp.bugs.scripts.checkwatches.core, line 571, in updateBugWatches
    server_time = remotesystem.getCurrentDBTime()
  Module lp.bugs.externalbugtracker.isolation, line 63, in wrapper
    return func(*args, **kwargs)
  Module lp.bugs.externalbugtracker.trac, line 305, in decorator
    self._authenticate()
  Module lp.bugs.externalbugtracker.trac, line 378, in _authenticate
    self.baseurl, '%s "%s"' % (error.code, error.msg))
BugTrackerAuthenticationError: http://gobby.0x539.de/trac: 401 "Unauthorized"