Do you get this with schedule backup? If yes do you get the same with client initiated backup? Check sbtio.log on client and daemon log on server for time when this happened to see if you get any additional log.

Well, this happens when using a scheduler named Control-M, which launchs the backup from the client.
Yesterday it worked fine. I'll try to see any other log if is posiible

What finally happened is that it was a physical problem when reading the tape. We used a different device and it worked fine. Thanks.