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.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability