After you deploy Cumulative Update 6 (CU6) for Microsoft Exchange Server 2013, you cannot manage Exchange Online mailboxes by using Exchange Admin Center (EAC) from an on-premises server. This is because some tasks are unavailable. For example, you can no longer do the following in Exchange Online:
Create a mailbox
Change a mailbox
Create or enable an archive mailbox
This problem affects only EAC. PowerShell cmdlets are not affected and continue to function correctly.
The problem occurs because of a missing configuration option in an EAC settings file.
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119591 How to obtain Microsoft support files from online services
Microsoft scanned this file for viruses. Microsoft used the most current virus-detection software that was available on the date that the file was posted. The file is stored on security-enhanced servers that help prevent any unauthorized changes to the file.
On the affected server that is running Exchange Server 2013, this script changes the EAC configuration option and restarts Internet Information Services (IIS). During the IIS restart process, users who are connected to the server that is being updated are disconnected briefly.