Thanks Pete! The former will explain the situation for us - the encryption key was changed on these files a couple of weeks ago.
Perhaps this is something that ought to be noted in the UI when changing/setting an encryption key ie. if you set or change the encryption key, you will manually need to delete any back up files that used the old key because of this issue of not being able to read the headers.
And probably something in the log file that also points to the issue and thorws a warning ie. "Warning xxxx file was not able to be deleted as encryption key does match the key currently stored on the job"
Perhaps this is something that ought to be noted in the UI when changing/setting an encryption key ie. if you set or change the encryption key, you will manually need to delete any back up files that used the old key because of this issue of not being able to read the headers.
And probably something in the log file that also points to the issue and thorws a warning ie. "Warning xxxx file was not able to be deleted as encryption key does match the key currently stored on the job"