Unknown error occurred for sync with google

Not sure if it's related to for example #517 but sync fails for certain items for me when doing a one-way sync from local vdir to google. The error is:

Copying (uploading) item 02jeahhru4b1lnvv4bpd7padjb_R20200113T160000@google.com to push_google_calendar_remote/272a302230d82590df68e0d235154648d0eb826d16a1b87178edf449e54d36e2@group.calendar.google.com
error: Unknown error occurred for push_google_calendar/272a302230d82590df68e0d235154648d0eb826d16a1b87178edf449e54d36e2@group.calendar.google.com: 400, message='Bad Request', url=URL('https://apidata.googleusercontent.com/caldav/v2/272a302230d82590df68e0d235154648d0eb826d16a1b87178edf449e54d36e2@group.calendar.google.com/events/6b654ddc-65c5-4743-983e-3dc72bc44e07.ics')

I have the following config:

[general]
status_path = "~/.vdirsyncer/status/"

[pair push_google_calendar]
a = "push_google_calendar_remote"
b = "push_google_calendar_local"
collections = ["272a302230d82590df68e0d235154648d0eb826d16a1b87178edf449e54d36e2@group.calendar.google.com"]
conflict_resolution = "b wins"

[storage push_google_calendar_remote]
type = "google_calendar"
token_file = "~/.vdirsyncer/token"
client_id = "xxx"
client_secret = "xxx"

[storage push_google_calendar_local]
type = "filesystem"
path = "~/.calendars/google"
fileext = ".ics"
read_only = true

and one of the entries it fails for looks as follows:

BEGIN:VCALENDAR
X-WR-CALNAME:xxx
X-WR-TIMEZONE:Europe/Stockholm
X-WR-CALDESC:xxx
VERSION:2.0
PRODID:-//Google Inc//Google Calendar 70.9054//EN
CALSCALE:GREGORIAN
BEGIN:VEVENT
RRULE:FREQ=WEEKLY;UNTIL=20200119T225959Z;BYDAY=FR,MO,TH,TU,WE
CREATED:20200109T094058Z
SEQUENCE:0
DTSTAMP:20200109T094308Z
DTEND:20200113T180000Z
LAST-MODIFIED:20200109T094308Z
DTSTART:20200113T160000Z
STATUS:CONFIRMED
SUMMARY:xxx: yyy
TRANSP:OPAQUE
UID:02jeahhru4b1lnvv4bpd7padjb_R20200113T160000@google.com
END:VEVENT
END:VCALENDAR

where I've replaced sensitive information with xxx or yyy. Note that the name/summary is "xxx: yyy", but I assume that's not an issue as other entries with : in the summary works.

Any ideas of what might be the issue, or how to get more details about what is actually bad about the request?

Start by reproducing the one-way local-to-Google Calendar sync with the provided configuration and failing recurring event. Inspect the Google CalDAV request and its 400 response to determine which event data is rejected; done means identifying the cause and either correcting the handling or reporting a useful actionable error.