I had this question after viewing Host Credentials for Oracle Enterprise Manager 11g not working?
Why does Backup through OEM not work in spite of correct credentials? It shows error message that required privilege is not held by the client.
I have already checked local policy settings as shown in some of the links, and also checked ORA_DBA group, etc.
The thing is that backup had been working fine for a long time and it suddenly started failing this month. Our network admin says nothing else has changed.