Have you tried refreshing the access token? Some OAuth 2.0 flows require using refresh tokens to acquire new access tokens as they have limited lifetimes to enhance security [1]. A refresh token will allow your application access Cloud SQL beyond the access token’s lifetime [2].
Based on the error message, you can have a look at these documentations about troubleshooting Cloud SQL connection issues [3][4]. Then by means of ensuring proper configuration I would follow the guide in the fifth and sixth links [5][6]. The last link is an answer on StackOverflow that I found [7]. Hope this helps.
[1] https://cloud.google.com/storage/docs/json_api/v1/how-tos/authorizing#OAuth2Authorizing
[2] https://cloud.google.com/storage/docs/json_api/v1/how-tos/authorizing#OAuth2Authorizing
[3] https://cloud.google.com/sql/docs/mysql/diagnose-issues
[4] https://cloud.google.com/sql/faq#connections
[5] https://cloud.google.com/sql/docs/mysql/connect-admin-proxy