I think that you will find that this is going to be more of an VSS (Volume Shadow Copy) error than with Data Protector
On your Exchange server, run
vssadmin list writers
See what the output shows. If there are any writers showing a State of "Failed" and the Last error stating "Retryable error" or "Non-Retryable Error", If the Last error states "Retryable error", then reboot the server and retry the backup operation, see how it goes
I also recommend that you check the Application Event Log and the System Event log on the Exchange server, perhaps, run another ebackup to see what errors are produced. Each error should have a identifiable code that you can do a 'google' search for
As a matter of fact,if you 'google' "Microsoft Exchange Writer could not prepare files for backup", you'll find quite a lot