Error 403 when syncing to Google Calendar

• Your vdirsyncer version: 0.16.8 (installed with pip)
• If applicable, which server software (and which version) you're using: google calendar, and an ICS from Office 365
• Your Python version: Python 3.8.5
• Your operating system: Debian sid
• Your config file:

status_path = "./status"

[storage o365]
type = "http"
url = "https://outlook.office365.com/owa/calendar/xx/xx/xx/reachcalendar.ics"

[storage gcal]
type = "google_calendar"
token_file = "gcal_token"
client_id="CLIENT_ID"
client_secret="CLIENT_SECRET"

[pair sync_o365_gcal]
a = "o365"
b = "gcal"
collections = [["calendar", null , "identifier@group.calendar.google.com"]]

Debug:

Copying (updating) item 9f50c6c1842c89541928ddb9fe8068bfb6c19c905e573dae75ffb10fc8377e9d to gcal/id@group.calendar.google.com
debug: Already normalized: '/caldav/v2/id@group.calendar.google.com/events/9f50c6c1842c89541928ddb9fe8068bfb6c19c905e573dae75ffb10fc8377e9d.ics'
debug: PUT https://apidata.googleusercontent.com/caldav/v2/id@group.calendar.google.com/events/9f50c6c1842c89541928ddb9fe8068bfb6c19c905e573dae75ffb10fc8377e9d.ics
debug: {'User-Agent': '973737131245-15cckkfme2da386qmvjkt74bic3gfk50.apps.googleusercontent.com', 'Content-Type': 'text/calendar', 'If-Match': '"63734152930"'}
debug: b'BEGIN:VCALENDAR\r\nPRODID:Microsoft Exchange Server 2010\r\nVERSION:2.0\r\nX-WR-CALNAME:Calendar\r\nBEGIN:VEVENT\r\nRRULE:FREQ=WEEKLY;UNTIL=20201101T230000Z;INTERVAL=3;BYDAY=MO;WKST=SU\r\nSUMMARY:Mec\r\nDTSTART;VALUE=DATE:20200831\r\nDTEND;VALUE=DATE:20200905\r\nCLASS:PUBLIC\r\nPRIORITY:5\r\nDTSTAMP:20200826T195637Z\r\nTRANSP:TRANSPARENT\r\nSTATUS:CONFIRMED\r\nSEQUENCE:0\r\nLOCATION:\r\nX-MICROSOFT-CDO-APPT-SEQUENCE:0\r\nX-MICROSOFT-CDO-BUSYSTATUS:FREE\r\nX-MICROSOFT-CDO-INTENDEDSTATUS:BUSY\r\nX-MICROSOFT-CDO-ALLDAYEVENT:TRUE\r\nX-MICROSOFT-CDO-IMPORTANCE:1\r\nX-MICROSOFT-CDO-INSTTYPE:1\r\nX-MICROSOFT-DONOTFORWARDMEETING:FALSE\r\nX-MICROSOFT-DISALLOW-COUNTER:FALSE\r\nUID:9f50c6c1842c89541928ddb9fe8068bfb6c19c905e573dae75ffb10fc8377e9d\r\nEND:VEVENT\r\nBEGIN:VEVENT\r\nRECURRENCE-ID;TZID=Romance Standard Time:20201012T000000\r\nSUMMARY:Mec\r\nDTSTART;VALUE=DATE:20201015\r\nDTEND;VALUE=DATE:20201017\r\nCLASS:PUBLIC\r\nPRIORITY:5\r\nDTSTAMP:20200826T195637Z\r\nTRANSP:TRANSPARENT\r\nSTATUS:CONFIRMED\r\nSEQUENCE:0\r\nLOCATION:\r\nX-MICROSOFT-CDO-APPT-SEQUENCE:0\r\nX-MICROSOFT-CDO-BUSYSTATUS:FREE\r\nX-MICROSOFT-CDO-INTENDEDSTATUS:BUSY\r\nX-MICROSOFT-CDO-ALLDAYEVENT:TRUE\r\nX-MICROSOFT-CDO-IMPORTANCE:1\r\nX-MICROSOFT-CDO-INSTTYPE:3\r\nX-MICROSOFT-DONOTFORWARDMEETING:FALSE\r\nX-MICROSOFT-DISALLOW-COUNTER:FALSE\r\nUID:9f50c6c1842c89541928ddb9fe8068bfb6c19c905e573dae75ffb10fc8377e9d\r\nEND:VEVENT\r\nBEGIN:VTIMEZONE\r\nTZID:Romance Standard Time\r\nBEGIN:STANDARD\r\nDTSTART:16010101T030000\r\nTZOFFSETFROM:+0200\r\nTZOFFSETTO:+0100\r\nRRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10\r\nEND:STANDARD\r\nBEGIN:DAYLIGHT\r\nDTSTART:16010101T020000\r\nTZOFFSETFROM:+0100\r\nTZOFFSETTO:+0200\r\nRRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3\r\nEND:DAYLIGHT\r\nEND:VTIMEZONE\r\nBEGIN:VTIMEZONE\r\nTZID:GMT Standard Time\r\nBEGIN:STANDARD\r\nDTSTART:16010101T020000\r\nTZOFFSETFROM:+0100\r\nTZOFFSETTO:+0000\r\nRRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10\r\nEND:STANDARD\r\nBEGIN:DAYLIGHT\r\nDTSTART:16010101T010000\r\nTZOFFSETFROM:+0000\r\nTZOFFSETTO:+0100\r\nRRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3\r\nEND:DAYLIGHT\r\nEND:VTIMEZONE\r\nEND:VCALENDAR'

debug: Sending request...
debug: 403
debug: {'Cache-Control': 'no-cache, no-store, max-age=0, must-revalidate', 'Pragma': 'no-cache', 'Date': 'Wed, 26 Aug 2020 19:55:26 GMT', 'Content-Type': 'text/xml; charset=UTF-8', 'Expires': 'Mon, 01 Jan 1990 00:00:00 GMT', 'Vary': 'Origin, X-Origin, Referer', 'Content-Encoding': 'gzip', 'Server': 'ESF', 'X-XSS-Protection': '0', 'X-Frame-Options': 'SAMEORIGIN', 'X-Content-Type-Options': 'nosniff', 'Alt-Svc': 'h3-29=":443"; ma=2592000,h3-27=":443"; ma=2592000,h3-T050=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"', 'Transfer-Encoding': 'chunked'}
debug: b'<?xml version="1.0" encoding="UTF-8"?>\n<D:error xmlns:D="DAV:"/>\n'
error: Unknown error occurred for sync/calendar: 403 Client Error: Forbidden for url: https://apidata.googleusercontent.com/caldav/v2/id@group.calendar.google.com/events/9f50c6c1842c89541928ddb9fe8068bfb6c19c905e573dae75ffb10fc8377e9d.ics
error: Use `-vdebug` to see the full traceback.
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/vdirsyncer/cli/utils.py", line 72, in handle_cli_error
debug:     raise e
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/vdirsyncer/sync/__init__.py", line 148, in sync
debug:     action.run(
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/vdirsyncer/sync/__init__.py", line 176, in run
debug:     self._run_impl(a, b)
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/vdirsyncer/sync/__init__.py", line 227, in _run_impl
debug:     self.dest.storage.update(meta.href, self.item, meta.etag)
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/vdirsyncer/storage/base.py", line 13, in inner
debug:     return f(self, *args, **kwargs)
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/vdirsyncer/storage/base.py", line 13, in inner
debug:     return f(self, *args, **kwargs)
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/vdirsyncer/storage/base.py", line 13, in inner
debug:     return f(self, *args, **kwargs)
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/vdirsyncer/storage/dav.py", line 544, in update
debug:     href, etag = self._put(self._normalize_href(href), item, etag)
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/vdirsyncer/storage/dav.py", line 514, in _put
debug:     response = self.session.request(
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/vdirsyncer/storage/dav.py", line 388, in request
debug:     return http.request(method, url, session=self._session, **more)
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/vdirsyncer/http.py", line 162, in request
debug:     r.raise_for_status()
debug:   File "/home/devilkin/.local/lib/python3.8/site-packages/requests/models.py", line 941, in raise_for_status
debug:     raise HTTPError(http_error_msg, response=self)
error: 1 out of 2 tasks failed.

This looks like #692

Reproduce the supplied configuration and trace the failed update from vdirsyncer/sync/init.py through storage/dav.py and http.py, especially the PUT request. Compare the behavior with issue #692 and determine why Google Calendar rejects this event; done means the provided sync completes without the 403 failure.