You have some third party tool backing up the databases as well as the native backups. My guess would be backupexec. The native and the 3rd part backups are interfering with each other causing this error.
Ensure that you have native backups or 3rd party database backups, not a mix of both. Either will work fine, a mix won't.
I'm pretty sure that at a previous place we had a huge database FULL backup directly to tape with BackupExec, and on occasion a native DIFF backup to disk without problems. Why would 2 different methods of full backup cause a problem ?
I've seen several cases where, for whatever reason, BackupExec invalidates the differential base. Haven't investigated why as I don't have it installed.
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