When you encounter a 403 Forbidden error, it may be due to a firewall configuration or the way your App Engine configuration file is set up.

(1) Try to check the firewall settings by navigating to your App Engine’s Firewall UI. Use Test IP Address to verify that the settings are correct and if the external IP would be allowed by the existing firewall rules.

(2) Check your app’s configuration files, specifically the app.yaml file, to see and check if the runtime.js file is correctly declared as static_files and not mistakenly marked as an application code file.

You can also check this StackOverflow post, which I think might be related to your concern but it offers a different workaround.

If above doesn’t not work and if you have a support package, I would recommend you getting help through reaching out to Google Cloud Support for a more in-depth analysis of your issue or you can just open an issue report on Google’s App Engine public issue tracker but please note that they won’t be able to provide a specific timeline for when the issue will be resolved.