I have experienced same problem with library. One time I am told to use overwrite scratch first and then it is changed again by support. I am not very happy with this product with Libraries.
This week it would not pull any media and cancelled the job because it wanted to tell me I would need new media soon. This media only used 5 times. I just don't seem to be able to get reliable backups.
Did they tell you about partioning the Library. I would try that if I ever felt confident with just doing backup from the server's harddrive.
Good luck.