Unfortunately this Update in is facing issues. Some customers found two issues with Exchange 2013 CU6:
- Exchange Server 2013 databases unexpectedly fail over in a co-existence environment with Exchange Server 2007 – http://support.microsoft.com/kb/2997209
- Exchange Online mailboxes cannot be managed by using EAC after you deploy Exchange Server 2013 CU6 –http://support.microsoft.com/kb/2997355
Both issues were quickly identified by Microsoft and a fix is already available.