Correct URL is here: https://forums.bitfire.at/post/9602
Synchronization with Google CalDAV/CardDAV is not officially supported by Google or DAVx⁵. For some people it seems to work, for others it doesn’t. Google officially requires OAuth 2, which is not supported by DAVx⁵.
We have received updated information about Google CalDAV:
The old endpoint https://www.google.com/calendar/dav is deprecated and no longer supported.
Now you shout suggest https://apidata.googleusercontent.com/caldav/v2/calid/events
Please have a look on https://developers.google.com/calendar/caldav/v2/guide
Google Account security:
If you have further information, please just post here.