Starting from patch#3, we're allocating some space on medium for our metadata in order to avoid situation when tape drive completely fills a tape, leaving no space for us. However, this happens only if device doesn't send us a corresponding warning about space running out. Also, the amount of allocated space is hard coded and equal to 2 GB. Therefore, the problem with left space exceeding this limit seems to be related to the tape device, rather than VB&R. For some reason it notifies us that currently inserted cassette is full, and we stop our operations.
You can open a ticket on this issue and double check it with the support team, though.
Also, haven't you updated your device recently? There is a similar case with one of our customers who has updated device drivers and started experiencing the said issue.