macOS Calendar app stopped syncing with Google Calendar
Deleting the Calendar cache and rebooting my Mac seemed to solve it, at least for now. I followed these instructions:

https://wilkinsit.ca/mac-osx/clear-calendar-cache-mac-os/


Backup Calendar from the File &gt; Export &gt; Calendar Archive… menu option.
Close Calendar.app
Navigate to Library &gt; Calendar folder in Finder
Remove all files that start with Calendar Cache
Reboot
Open Calendar.app and watch as it initializes calendars