Inventory job or any tape jobs may fail with an error “Tape <tape media name> requires return to original slot <slot number>, but it is already filled. Move tape manually, and retry operation”
You may find following entries in the Job.<Job name>.TapeVmBackup.log / Job.<Job name>.TapeFilesBackup or Job.Tape_library_inventory.TapeInventory:
[timestamp] <01> Error Tape not exchanged. (Veeam.Tape.Core.TapeMediumNotExchangedException)
[timestamp] <01> Error at Veeam.Tape.Core.AutomatedTapeMediumExchanger.ExchangeTapeMediumsForBackup(Boolean updateMediaInfo)
[timestamp] <01> Error at Veeam.Tape.Core.TapeBackupTaskPerformerBase.BackupStartSession(ITapeBackupObject task)
[timestamp] <01> Error at Veeam.Tape.Core.VmTapeBackupTaskPerformer.Backup()
[timestamp] <01> Error Tape VeeamTest requires return to original slot 7, but it is already filled. Move tape manually, and retry operation. (System.Exception)
[timestamp] <01> Error at Veeam.Tape.Core.TapeMediumLoader.CalcTapeSlot(TapeMedium tapeMedium)
[timestamp] <01> Error at Veeam.Tape.Core.TapeMediumLoader.UnloadTapeFromDrive(ChangerHandle changerHandle, TapeMedium tapeMedium)
[timestamp] <01> Error at Veeam.Tape.Core.TapeMediumLoader.LoadTapeToDrive(TapeMedium tapeMedium, TapeDrive tapeDrive)
[timestamp] <01> Error at Veeam.Tape.Core.TapeMediumLoader.LoadTapeToDrive(TapeMedium tapeMedium, TapeDrive tapeDrive, IXmlLogger logger)
[timestamp] <01> Error at Veeam.Tape.Core.AutomatedTapeMediumExchanger.LoadMedium(TapeMedium neededTapeMedium)
[timestamp] <01> Error at Veeam.Tape.Core.AutomatedTapeMediumExchanger.LoadValidMediumForBackup(Boolean updateMediaInfo)
[timestamp] <01> Error at Veeam.Tape.Core.AutomatedTapeMediumExchanger.ExchangeTapeMediumsForBackup(Boolean updateMediaInfo)
[timestamp] <01> Error No media in drive (System.Exception)
[timestamp] <01> Error at Veeam.Tape.Core.TapeHandle.GetMediaInfo()
[timestamp] <01> Error at Veeam.Tape.Core.TapeJobPerformerContext.UpdateTapeMediaInfo()
[timestamp] <01> Error at Veeam.Tape.Core.VmTapeBackupTaskPerformer.Backup()
[timestamp] <01> Error at Veeam.Tape.Core.TapeBackupTaskPerformerBase.Execute()
Another tape media most likely occupies the original (home) slot of the tape media being used in the job, so you need to manually unload tape media and let job succeed on retry.