Error refreshing the OAuth2 token, message: '{ "error": "invalid_grant", "error_description": "Bad Request" }'
Error refreshing the OAuth2 token, message: '{ "error": "invalid_grant", "error_description": "Token has been expired or revoked." }
Those errors suggest that the plugin somehow has lost the authorization to access your Google Drive. For instance, this can happen if you change your password. This means that you have to re-authorizing the plugin again. To do so, just press the revoke button on the plugin Dashboard an reauthorize the plugin.
In some situation, the revoke button may not work as expected which requires you to manually reset the authorization. To do so, please remove the (SITE_ID).access_token file from the wp-content/use-your-drive-cache folder. If you are not running a MultiSite setup, the (SITE_ID) will be 1.
That will reset the authorization completely, and should fix the problem. If the problem persist, please create a support ticket!