What you need to know
A page can display a generic error for a wrong URL, blocked preflight request, invalid body, expired session, or server exception. The Network panel shows which layer failed.
How to apply it
Filter requests by Fetch/XHR, reproduce the action, inspect status and response, and compare the request shape with the backend contract. Check the server logs using a safe request ID if available.
Common mistake to avoid
Do not assume a CORS message means the API is down. Verify the actual request and server response before changing security headers.