There is definitely something wrong with the inventory. Check whether you have any outstanding alerts in BE and check the Windows Eventlog to see whether there are any exceptions. Cancel the inventory job and restart all the BE services.
If the problem persists, then run the manufacturer's diagnostic utility against the tape library. Make sure that you select the write test and that you have stopped all the BE services beforehand.
If you are using barcode labels, then scan the library. Scan does the same thing as inventory and it is very fast.