I've used ntbackup tapes with Veritas before without issue.
However, in seperate incidents, I have seen teh "urencognizable media" error before.
http://forums.veritas.com/discussions/thread.jspa?threadID=44212&tstart=0
Try to do a quick erase, if that doesn't resolve the issue, or if it comes back, you have my issue.
Solution: just accept it and move on with your life. After a billion hours of scouring the web, using these forums, and Veritas tech support, I haven't come across a solution, unless you consider requests for intimacy and large amounts of control characters a response.
