Interesting that you only saw the 85's with appends in the past.
I have seen the header issues with encryption problems. The problem is if the tape drive drive does not does not have the correct key, it can not read or write the netbackup header on the tape.
The key problem could be a problem with the tape library, or with the path used to transport the key from the kms to the tape drive.
Or a problem with the kms server.
Or trying to use a key that does not exit in the kms server.
Are you using netbackup key management of the keys or .....