If you suddenly discover backup files aren’t uploaded to Google Drive anymore, we recommend ensuring each WordPress installation is connected to a distinct Google Drive API project.
Remember that this doesn’t imply the need for multiple Google accounts. Rather, you should set up an independent project within the API console of your existing Google account. If you need a refresher on how to do this, refer to this detailed “Google API Credentials Setup” guide.
Also, ensure that all your website backups start at different times. This will reduce the risk that Google you will run into Google Usage Limits. At the writing of this article, Google allows 20.000 requests per 100 seconds.
You can use multiple websites with the same API credentials as long as not all scheduled backup runs simultaneously. Still, the exact number of websites depends on how big a website is and how many scheduled and periodic backups are created.
You could start with something like 10 websites per project, and if you still get authentication errors to Google Drive, you can create more Google API projects to reduce the number of requests per project even further.
So far, Google hasn’t officially acknowledged this issue or provided a solution within its documentation. Their official guides only highlight the necessity of a unique client ID for each WordPress installation. However, many users have experienced random access errors on Google Drive when using the same API project for several websites. This indicates a possible flaw in Google’s code.