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Force quit Calendar again. Then go to System Preferences > iCloud. Sign out from iCloud. Select Keep if asked to Delete or Keep. If you only see Delete, choose it. Then open Calendar and you will not get that error. Now add some entries. Close it.
Then go to System Preferences > iCloud and add the account back. Open Calendar
Let me know how it goes please
If it still not working, force Quit Calendar. Then go to System Preferences > iCloud and turn off Calendar and also Reminders. Sign out from iCloud. Open and close Calendar.
Then go to System Preferences > iCloud and sign in back to iCloud. Uncheck Reminders. Wait 2 minutes. Then open Calendar. If asked to Merge or Erase, choose Merge. If Calendar is working, then go to System Preferences > iCloud and enable Reminders back.
Then open Calendars. Please let me know if that didn't work, so we can continue troubleshooting