If you have an Exchange Hybrid Configuration setup, you may have some users that use services of Office365 but still have their mailbox on-premise. These users probably have an Office365 license assigned to them. Selecting these users from the Users list in the Office365 Admin Portal reveals the following message when viewing the primary e-mail address for such a user:

Error

The items you're trying to open couldn't be found.

Additionally, the same error is displayed when attempting to view or edit Exchange Online properties for such a user.

Cause

The error is actually a "bug" in the Office365 Admin Portal, caused because the user has an Exchange Online license assigned. Because of this, Office365 automatically assumes the user has an online mailbox rather than an on-premise mailbox. Because the hybrid configuration prevents the creation of double mailboxes (i.e. Exchange Online is aware the user has on-premise mailbox), no mailbox is created on Exchange Online, but the Admin Portal is not aware of this fact, and cannot find the mailbox linked to the user, precisely because it's an on-premise mailbox.

Workaround

To work around the issue, edit the license for the user and unselect Exchange Online:

The primary e-mail address for the user will now say the user does not have an Exchange mailbox (which is somewhat true as the user does not have an Exchange Online mailbox). The links to view/edit the Exchange Online properties for the user will have been removed.