Start with the URL itself: confirm it is current, uses HTTPS and is intended for browser access. Then distinguish a manifest failure from a segment failure. A 403 often points to authorization or an expired signed URL, while 404 indicates a missing resource. A CORS failure must be corrected by the server that hosts the stream.
Practical advice
Use the original source file or URL whenever possible, keep a backup before conversion, and treat browser error messages as troubleshooting signals rather than proof of a specific server-side configuration.