If you have 3rd party apps accessing your Google Calendar, you may need to reset your Google Calendar Secret Address and paste it into that app.
- Open Google Calendar
- In the left-hand column, hover over the calendar you need to access
- Click the three dots beside the calendar and select Settings and sharing
- In the left-hand column, click on Integrate Calendar
- In the Secret address in iCal format section, click the Reset button
- Click the Copy to Clipboard icon to copy the address for use in other applications