When the Microsoft Outlook Web App logon page or the Exchange Admin Center (EAC) logon page is displayed in Spanish in a Microsoft Exchange Server 2013 environment, the label for the username is Nombre de usuario\dominio: instead of Dominio\nombre de usuario:. Therefore, users cannot log on to Outlook Web App or EAC.
The issue occurs because the label is translated incorrectly.
To resolve this issue, install the following cumulative update:
2892464 Description of Cumulative Update 3 for Exchange Server 2013
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.