Today I ran into the issue described on this Veeam Community Forum post. Under specific circumstances, Veeam inadvertently marks a legitimate tape as a cleaning tape, removing it entirely from view in VBR console. There's no user-accessible way to remove it from the catalog, nor does erasing, re-scanning, or inventorying the tape have any effect.
Symptoms:
You will see the following entries in your logs:
:: TapeDrive alert: The tape drive is due for routine cleaning:. Wait for the current operation to finish. Then use a cleaning cartridge. Check the tape drive user manual for device specific cleaning instructions.
:: TapeDrive alert: The last cleaning cartridge used in the tape drive has worn out:. Discard the worn out cleaning cartridge. Wait for the current operation to finish. Then use a new cleaning cartridge.
:: Marking tape <TapeName> as a cleaning cartridge
And you will notice a slot in your tape library has magically disappeared. The missing slot will correspond with the location of the tape Veeam believes is a cleaning cartridge.