I have a SharePoint Add-In application that has been running for fine for a while with on coding changes and yesterday an error suddenly started to happen when attempting to get an app-only token:
Exception: Token request failed.
Inner Exception: The remote server returned an error: (401) Unauthorized.
The client ID/secret doesn't expire until next year. The application is running on .Net 4.7.2 and should be using TLS 1.2.
What could be the problem?