Hello,
Does the new account have Administrator rights? If not, ensure that you use an account with Admin rights. The backup exec services too should use the same account. To backup Exchange, the account must have Exchange full admin rights.
Refer to the following technote in order to resolve the problem:
Troubleshooting basic "access denied" and "access is denied" messages in Backup Exec 9.x and 10.0 for Windows Servers
http://support.veritas.com/docs/266075
Also check the needed rights in order to backup Exchange:
How to configure a Microsoft Exchange 5.5 and 2000 individual mailbox backup for Backup Exec 8.x, 9.x or 10.x software
http://support.veritas.com/docs/249456
Hope this helps. If the error persists, revert with details.
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Regards.