I think either there's an issue with token used by Azure App Service Creates or the source control property of your app service i.e., the Deployment Center blade, became corrupt. You can view these settings using https://resources.azure.com and navigating to your app service resource or az webapp show --name <app name>
and inspect the sourceControls property.
I suggest unlinking the existing deployment connection and reconnecting the GitHub repo. If it still continues to fail, revoke and remove the OAuth app so that when you reinitiate the connection from the Azure portal, you'll be re-prompted to authorize the app and it should get a new token.