In some earlier versions of NetBackup there was a bug where in some cases all Oracle backups would end with status code 6, but it's been fixed by v6.5.|
As the others have written, there may be detail in the child jobs or you may have to use logging that your RMAN script has created. For certain backups, I've seen a status code 6 when RMAN was told to backup archive redo logs and a quiet DB has created none.
If this isn't an agent backup, then it might be something like a policy specifying (only) an object that does not exist on the client.
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