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.