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.

The error is actually a "bug" in the Office365 Admin Portal, caused because the user has an Exchange Online license assigned. Because of thi  ...

Setting up a hybrid configuration between Office 365 and on-premise Exchange may seem straight forward, but there are a lot of pitfalls to tackle. One of the more frequent issues is a problem with free/busy information not being visible, or the inability to migrate mailboxes from/to Exchange Online.

Additionally, you may have received this error during the initial configuration of the hybrid setup:

Hybrid Configuration Wizard
Office 365 was unable to communicate with your on-premises Autodiscover endpoint. This is typically due to incorrect DNS or firewall configuration. The Office 365 tenant is currently configured to use the following URL for Autodiscover queries from the Office 365 tena  ...

In an Office365 hybrid configuration, you may get the following error after the Sign in to Office 365 prompt in the EAC:

Error

412

Cookies Are Disabled :(

Please make sure that you enabled cookies in your browser settings and that your Exchange Admininstration Center domain has been added to trusted sites or local intranet zones.

To resolve, you need to do just that:

1. Go to Internet Options, tab Security.
2. Click the Intranet zone, then click the Websites button, then the Advanced button.
3. Add your EAC FQDN to the list (e.g. https://exchange01.contoso.com/).
4. Completely close the browser, then log back in.