If the synchronization between a Google calendar and your coapp resource no longer works, this can have several causes:
The calendar that the coapp resource was synced with no longer exists
The Google user who set up the synchronization no longer exists or does not have read and write permissions for the Google calendar with which the coapp resource was synchronized.
The coapp user who set up the sync no longer exists
Solution:
Remove the Google Calendar sync by editing the coapp resource and selecting "Select connected Account" in the "Select Account" section and "Select calendar" in the "Select Calendar" section.
Save your changes to the resource by clicking the "Update" button at the very bottom of the page.
Re-establish the connection between your resource and a Google Calendar. If necessary, add a new Google account whose calendar you want to use for synchronization. Otherwise, use an existing connection with a Google account. Here you can find instructions on how to set up Google Calendar synchronization with coapp.