Hi,
This may occur due to various reasons. here are some steps that needs to be taken care of inorder to backup the exchange mailboxes and public folders.
1. The account that you have created should have "The Exchange Full Administrator" Rights.
2. The account should have the "Domain admins" and the "Administrator" Privileges.
3. The account should also have an Unique mailbox.
4. The mailbox should be active.
5. The account should not be hidden from the "Global Address list".
You may refer the following technotes for reference,
http://seer.support.veritas.com/docs/242606.htm
http://seer.support.veritas.com/docs/262135.htm
-You may also refer the following technotes,
http://seer.support.veritas.com/docs/262707.htm
http://seer.support.veritas.com/docs/265533.htm
Hope this helps.