Per Gr-R's helpful advice, there seemed to be filesystem corruption
Here's what needed to be done:

Disable Local Backup (Configuration -> NAS Settings -> disable Local Backup)
Delete the logical volume that contained the backup (any data on this LV WILL BE LOST!)
Fix the filesystem:
(From the console) Go to Extended Tools (Ctrl-Alt-X), Select "Repair filesystem on LV" (#9), Hit yes to continue, then REBOOT!

After this, I was able to label tapes.