If a GRT enabled full backup is run to tape and a GRT differential or incremental backup is run to disk, the differential or incremental backup may fail with a job error.
Final error: 0xe000fe21 - An invalid path or file name was specified.
Final error category: Job Errors
The data changes that make up a Sharepoint incremental or differential backup are combined with the full backup to create a composite view of the data. When the full and incremental/differential backups are targeted to different storage device types this combination step is not working properly.
FULL backup of the SharePoint Farm is done to tape. Differential or incremental backup to a disk target fails. If the differential or incremental runs to tape, it completes successfully.