After Microsoft released Cumulative Update 6 for Exchange Server 2013 last week, a bug was discovered by Jeff Guillet as for Exchange Hybrid customers and their ability to manage Online mailboxes by using EAC.
Today Microsoft released a new support KB that describe the problem:
Exchange Online mailboxes cannot be managed by using EAC after you deploy Exchange Server 2013 CU6
The support KB is followed by a script which fixes the described issue and can be download from the following link:
Exchange Online mailboxes cannot be managed in Exchange Server 2013 CU6
After you install Microsoft Exchange Server 2013 Cumulative Update 6, the Hybrid node in Exchange Admin Center (EAC) no longer functions correctly. This prevents administrators from being able to administer Exchange Online Mailboxes from an On-Premises server.
Please be aware that the fix is resting the IIS service, therefore it is highly recommended not to apply it during business hours.
Another fix that Microsoft released is for Co-existence between Exchange 2013 and Exchange 2007 but unfortunately it requires you to contact Microsoft support for the hotfix:
Exchange Server 2013 databases unexpectedly fail over in a co-existence environment with Exchange Server 2007
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