process-mail.py OOPSes with IllegalTarget

Trying out the e-mail interface as described in https://help.launchpad.net/Bugs/EmailInterface always failed with an OOPS.

OOPS-2103INBOUNDEMAILEMAIL1
OOPS-2102INBOUNDEMAILEMAIL1

Traceback from the OOPS:

Traceback (most recent call last):
  Module lp.services.mail.incoming, line 398, in handleMail
    signature_timestamp_checker)
  Module lp.services.mail.incoming, line 504, in handle_one_mail
    handled = handler.process(mail, email_addr, file_alias)
  Module lp.bugs.mail.handler, line 288, in process
    bug, bug_event)
  Module lp.bugs.mail.commands, line 621, in execute
    bugtask = self._create_bug_task(bug, bug_target)
  Module lp.bugs.mail.commands, line 683, in _create_bug_task
    return self._targetBug(user, bug, bug_target)
  Module lp.bugs.mail.commands, line 652, in _targetBug
    general_task = bug.addTask(user, general_target)
  Module lp.bugs.model.bug, line 1192, in addTask
    new_task = getUtility(IBugTaskSet).createTask(self, owner, target)
  Module lp.bugs.model.bugtask, line 2719, in createTask
    validate_new_target(bug, target)
  Module lp.bugs.model.bugtask, line 427, in validate_new_target
    "been reported." % target.displayname)
IllegalTarget: This bug is already on Ubuntu. Please specify an affected package in which the bug has not yet been reported.