webdavbackend.py: expected bytes-like object, not str

A long running script now returns an error. It seems to be that webdavbackend.py does not conform to newest Python standards. I am using duplicity 8.04. My webdav user name is '<email address hidden>' - entering in Bash with double quotes worked until now, now I entered michael.haelsig\@online.de. Both variants fail.

duplicity $version ($reldate)
Args: /usr/bin/duplicity --verbosity debug --volsize=100 /home/mhaelsig/Skripte webdavs://<email address hidden>:<email address hidden>/Test
Linux Debian9 5.2.0-2-amd64 #1 SMP Debian 5.2.9-2 (2019-08-21) x86_64
/usr/bin/python3 3.7.4+ (default, Sep 4 2019, 08:03:05)
[GCC 9.2.1 20190827]
================================================================================
Temporäres Verzeichnis /tmp/duplicity-tik_134h-tempdir wird benutzt
(mkstemp) temporäre Datei /tmp/duplicity-tik_134h-tempdir/mkstemp-ajccro_7-1 wird registriert
9445765120 temporärer Speicherplatz ist verfügbar, die Sicherung wird ungefähr 136314880 nutzen.
WebDAV erzeugt eine Verbindung unter 'sd2dav.1und1.de'
WebDAV PROPFIND /Test/ Anfrage mit Headern: {'Connection': 'keep-alive', 'Depth': '1'}
WebDAV Datenlänge: 95
WebDAV Antwortstatus 401 mit dem Grund 'Unauthorized'.
Rückverfolgung des vorangegangenen Fehlers: Traceback (innermost last):
  File "/usr/lib/python3/dist-packages/duplicity/backend.py", line 371, in inner_retry
    return fn(self, *args)
  File "/usr/lib/python3/dist-packages/duplicity/backend.py", line 580, in list
    return [tobytes(x) for x in self.backend._list()]
  File "/usr/lib/python3/dist-packages/duplicity/backends/webdavbackend.py", line 343, in _list
    raise e
  File "/usr/lib/python3/dist-packages/duplicity/backends/webdavbackend.py", line 317, in _list
    response = self.request(u"PROPFIND", self.directory, self.listbody)
  File "/usr/lib/python3/dist-packages/duplicity/backends/webdavbackend.py", line 240, in request
    self.headers[u'Authorization'] = self.get_authorization(response, quoted_path)
  File "/usr/lib/python3/dist-packages/duplicity/backends/webdavbackend.py", line 271, in get_authorization
    return self.get_basic_authorization()
  File "/usr/lib/python3/dist-packages/duplicity/backends/webdavbackend.py", line 291, in get_basic_authorization
    return u'Basic %s' % base64.encodestring(auth_string).strip()
  File "/usr/lib/python3.7/base64.py", line 540, in encodestring
    return encodebytes(s)
  File "/usr/lib/python3.7/base64.py", line 527, in encodebytes
    _input_type_check(s)
  File "/usr/lib/python3.7/base64.py", line 513, in _input_type_check
    raise TypeError(msg) from err
 TypeError: expected bytes-like object, not str