RMAN backup of archivelogs fails:
ORA-19588: archived log recid xxxx stamp xxxxxxxx is no longer valid
The rman backup job had been started TWICE.
The initial resync for each job identified the same set of logs to be backed up.
Both jobs were trying to backup and delete the SAME archivelog set, one of which failed eventually as the other job had already backed up and deleted the log.
Make sure that concurrent backups of the archivelogs are not run.