If a Google calendar connection is disconnected, permissions for the Google calendar may need to be reset before that calendar can be connected again.

While logged into the Google account that owns the calendar you are trying to reconnect, this ensures you are working with an account authorized to manage the calendar.

With the connection removed, it is now possible to reconnect the Google account and reconnect the calendar.

Additionally, confirm that you have the appropriate permissions for the calendar, such as 'Make Changes and manage sharing'.