Under a certain number, chapters don't load

In every manga (i suppose, i didn't check them all) under a number of chapter i get:

  File "/opt/extras.ubuntu.com/mangar/mangar/MangarWindow.py", line 108, in my_on_episodetreeview_row_activated
    self.download_image(episodes_url, manga, episode_number, i)
  File "/opt/extras.ubuntu.com/mangar/mangar/MangarWindow.py", line 544, in download_image
    data = self.get_page_source(url)
  File "/opt/extras.ubuntu.com/mangar/mangar/MangarWindow.py", line 292, in get_page_source
    usock = urllib2.urlopen(page)
  File "/usr/lib/python2.7/urllib2.py", line 126, in urlopen
    return _opener.open(url, data, timeout)
  File "/usr/lib/python2.7/urllib2.py", line 406, in open
    response = meth(req, response)
  File "/usr/lib/python2.7/urllib2.py", line 519, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python2.7/urllib2.py", line 444, in error
    return self._call_chain(*args)
  File "/usr/lib/python2.7/urllib2.py", line 378, in _call_chain
    result = func(*args)
  File "/usr/lib/python2.7/urllib2.py", line 527, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
urllib2.HTTPError: HTTP Error 404: Not Found