Logged in as you, using your Outlook profile you can add this mailbox with out issue?

The reason I ask is because this is a common response/error if you do not have permissions to a mailbox.

Another possibility is that the mailbox is offline and can not communicate with the Exchange server. Check this on Outlook by going to File - Offline, is there a check mark next to it?

If the above does not resolve the issue then the issue maybe with Outlook itself or the profile. I would fist create a new profile for your self and test it to see if you can then add the additional mailbox

Kind Regards,


--
Francine Otterson
Microsoft Outlook MVP
President, San Diego Outlook User Group