unkown error 403 on vdirsyncer sync after task creation

[Konfekt]

Before you submit bug reports: https://vdirsyncer.pimutils.org/en/stable/contributing.html

Things to include in your bugreport:

• Your vdirsyncer version
  0.16.6
• If applicable, which server software (and which version) you're using
• Your Python version
  2.7.15 and 3.6.5
• Your operating system
  OpenSUSE 15
• Your config file

[general]
# A folder where vdirsyncer can store some metadata about each pair.
status_path = "~/.local/share/vdirsyncer/status/"

[pair restCalendar]
a = "restCalendarLocal"
b = "restCalendarRemote"
# collections = ["private", "work"]
collections = ["from b"]

# Calendars also have a color property
metadata = ["rest", "blue"]

# To resolve a conflict the following values are possible:
#   `null` - abort when collisions occur (default)
#   `"a wins"` - assume a's items to be more up-to-date
#   `"b wins"` - assume b's items to be more up-to-date
conflict_resolution = "a wins"

[storage restCalendarLocal]
type = "filesystem"
path = "~/.local/share/vdirsyncer/calendars/user/"
fileext = ".ics"

[storage restCalendarRemote]
type = "caldav"
url = "https://spica.t-online.de"
username = "user@t-online.de"
password.fetch = ["command", "pass", "sync/rest"]

• Use vdirsyncer -vdebug for debug output. The output is sensitive, but
  please attach at least the last few lines before the error (if applicable),
  censored as necessary. This is almost always the most useful information.

If there is a single task test in the list whose content is

BEGIN:VCALENDAR
VERSION:2.0
PRODID:io.barrera.todoman
BEGIN:VTODO
CREATED;VALUE=DATE-TIME:20181109T002041Z
DTSTAMP;VALUE=DATE-TIME:20181109T002041Z
DUE;VALUE=DATE-TIME:20181111T002000Z
LAST-MODIFIED;VALUE=DATE-TIME:20181109T002048Z
SEQUENCE:1
STATUS:NEEDS-ACTION
SUMMARY:test
UID:68049bd6662e456096a8a3479368055e@user-pc
END:VTODO
END:VCALENDAR

then vdirsyncer -v DEBUG sync gives

debug: Sending request...
debug: 403
debug: {'Date': 'Fri, 09 Nov 2018 00:21:50 GMT', 'Server': 'Apache', 'X-Generated-By': 'spica06.aul.t-online.de', 'X-Request-ID': 'a1469177-6694-48e7-86ad-649c6b4b93c0', 'X-Content-Type-Options': 'nosniff', 'X-XSS-Protection': '1; mode=block', 'Cache-Control': 'no-cache, no-store, max-age=0, must-revalidate', 'Pragma': 'no-cache', 'Expires': '0', 'Strict-Transport-Security': 'max-age=31536000 ; includeSubDomains', 'X-Frame-Options': 'DENY', 'Content-Length': '0', 'Keep-Alive': 'timeout=5, max=99', 'Connection': 'Keep-Alive', 'Content-Type': 'text/calendar'}
debug: b''
error: Unknown error occured for restCalendar/ADDRESS_BOOK: 403 Client Error:  for url: https://spica.t-online.de/spica-calendar/caldav/principals/user@t-online.de/calendars/ADDRESS_BOOK/120049010000000065340288-120049010000000065340288_2365622157_B.ics
error: Use `-vdebug` to see the full traceback.
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/cli/utils.py", line 74, in handle_cli_error
debug:     raise e
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/sync/__init__.py", line 153, in sync
debug:     partial_sync
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/sync/__init__.py", line 177, in run
debug:     self._run_impl(a, b)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/sync/__init__.py", line 228, in _run_impl
debug:     self.dest.storage.update(meta.href, self.item, meta.etag)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/storage/base.py", line 15, in inner
debug:     return f(self, *args, **kwargs)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/storage/base.py", line 15, in inner
debug:     return f(self, *args, **kwargs)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/storage/dav.py", line 546, in update
debug:     href, etag = self._put(self._normalize_href(href), item, etag)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/storage/dav.py", line 520, in _put
debug:     headers=headers
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/storage/dav.py", line 390, in request
debug:     return http.request(method, url, session=self._session, **more)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/http.py", line 163, in request
debug:     r.raise_for_status()
debug:   File "/home/epn/.local/lib/python3.6/site-packages/requests/models.py", line 935, in raise_for_status
debug:     raise HTTPError(http_error_msg, response=self)
error: 2 out of 6 tasks failed.

See also pimutils/todoman#334.

Any idea why this error message occurs? Strangely, the OpenTasks Android app informed that the t-online calendar permits adding tasks, but Thunderbird Lightning thinks differently. Who's right? Can a calendar be set to allow tasks?