OK, via the GUI - if you check for updates, you should find updates available for mhVTL.
If you still experiencing issues following the update, please set "VERBOSE = 3" in /etc/mhvtl/mhvtl.conf
Restart the mhVTL daemons
and re-try the Tape Library Inventory from Veeam.
Provide /var/log/kern.log and /var/log/syslog. The /var/log/syslog is the main one of interest.