The specified object was not found in the store.
The most common problem when encountering this error is that the admin account does not have the correct permissions to access the mailbox.
You can verify you that have the correct permissions by performing a mailbox access test. If the permissions are correct, you should be able to see the content of Inbox of the user; otherwise OWA will respond with error messages like "You do not have access to open this mailbox".
You may grant the administrator permission accordingly to solve the issue.
Mailbox Is Hidden From The Global Address List
If the Source is Exchange 2003, and the user is hidden from the global address list, make sure that prior to migration, the user is no longer hidden: