Following an upgrade to Veeam Backup & Replication 7.0 Patch 4, tape backups failed with the error “Unable to start tape backup session: StartSessionTape failed”.

In order to get the backups working again I upgraded the firmware on the tape library, which simultaneously upgraded the firmware on the tape drives.  I then upgraded the drivers for the tape library and the tape drives.  Following these steps the tape jobs ran successfully again.