Ran into an issue today where I had an exchange 2013 and 2010 server in the same domain. When I tried to log into the Exchange 2013 control panel it kept sending me to OWA. 

Have a look here:
http://technet.microsoft.com/en-us/library/jj150562.aspx

“If you’re in a coexistence scenario, where you’re running Exchange 2010 and Exchange 2013 in the same organization, and your mailbox is still housed on the Exchange 2010 Mailbox server, the browser will default to the Exchange 2010 ECP. You can access the EAC by adding the Exchange version to the URL. For example, to access the EAC whose virtual directory is hosted on the Client Access server CAS15-NA, use the following URL: https://CAS15-NA/ecp?ExchClientVer=15. Conversely, if you want to access the Exchange 2010 ECP and your mailbox resides on an Exchange 2013 Mailbox server, use the following URL: https://CAS14-NA/ecp?ExchClientVer=14.”

Try the URL modification:
https://<Exchange_2013_CAS_FQDN>/ecp?ExchClientVer=15

It will show up as OWA authentication but after you provide credentials it will redirect back to EAC.

 

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