This issue seems to have more than one cause, and several possible fixes have been reported. I can't personally verify any of them.
1. Disable and re-enable the problem calendar in the Calendars list.
2. Delete the calendar in the Accounts tab of Calendar preferences. Quit and relaunch the application. Add the calendar. Make sure you select the right account type; e.g., Google.
3. Delete calendars for which you're a delegate.
4. Quit Calendar. In the Finder, hold down the option key and select
Go ▹ Library
from the Finder menu bar. From the folder that opens, move these subfolders (some may not exist) to the Trash, leaving the Library folder open:
- Caches/com.apple.CalendarAgent
- Caches/com.apple.iCal
- Calendars/Calendar Cache
Relaunch Calendar and test.