Backing Up Exchange Server Using ARCserve

I have Exchange Server 2000 running one a server here, and ARCserve 7 running on another. Now, when ARCserve gets to backing up the individual mailboxes, the 'Backup Agent RPC Server' service stops, possibly during the backup as it seems to back up about 8 individual mailboxes. I've checked the error log and it says 'Backup agent error' for a few mailboxes, then 'Remote procedure call failed' for subsequent mailboxes.

Does any one with any experience of ARCserve have any idea why it's failing? :(

Thanks