Make sure the account which you are using have the following rights and permission
Exchange Full Administrator
Domain Admins/Explicit Local Admin
Has active mailbox
Has sent/received e-mail
Is Unique in AD (1st five character should not match with any other account )
Is not hidden in GAL
You can verify the account have proper rights By doing the following steps
go to Tools | Option |Ms exchange check the box for enable legacy mailbox support
then got back to you backup selection and try expanding the MS mailboxes in the selection . If you are able to expand the mailboxes then the account have propert rights and permission .
Also you can try creating a new account and give all the rights and permission