Bizarre message whenever I open my calendar
It is possible that you are trying to access a calendar link that was created before Google changed their URL formats (which was a long time ago). Old Google calendar links started with
https://www.google.com/calendar/dav/

Back in 2013, they changed the URL format to
https://apidata.googleusercontent.com/caldav/v2/your-email

I would suggest that firstly, you should delete any subscribed calendars (e.g. things like the &quot;Public Holidays&quot; calendar. If the problem persists, the next step would be to remove your account from the calendar app entirely, close the calendar app, then reopen and re-add your account details.
Make sure you've backed up your data first.