I have the same problem. I also have a Sony LIB-81
It began with BE 9.1 SP2 - before SP2, the shutter was locked by scsi only very occasionally, maybe 2 times in a year, and unlocking it did not cause the device to go offline. After SP2, the shutter is locked by scsi every time I try to replace tapes, and when I unlock it in Backup Exec, the device goes offline. Only a reboot restores the connection. Later I installed SP3 hoping that would fix this bug, but it did not.
I tried your suggestion to recreate the tape drives, no luck.
I do not want this functionality - can we turn off the locking of the shutter? I understand it is meant for my protection, but it is making my backups extremely unreliable and requiring a manual reboot every day.