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ORA-01260:

warning: END BACKUP succeeded but some files found not to be in backup mode
Cause: END BACKUP completed successfully for all files that were in online backup mode. However one or more modifiable files were found not to be in online backup mode. Backup of those files (if it was done) can be invalid and, if restored, can result in an inconsistent database.
Action: Check the alert log for a list of the files that were found not to be in backup mode. If there is a possibility that those files have been modified during online backup, then replace their backups with new ones.