Dataset content returns 409, not 403

A read that worked yesterday now returns 409. RACF did not change. What does 409 mean here?

409 means the dataset is exclusively open by another process. It is not a RACF deny and not a missing dataset. Those are 403 and 404. Ask who has the file open on z/OS, or retry when that holder closes it. Do not rotate credentials for a 409.