If you deploy Exchange 2013 within an environment with Exchange 2010 already deployed, the ECP (Exchange Control Panel) used is based on the Exchange version where your mailbox is hosted.
So if Exchange Administrator has mailbox hosted on Exchange 2010, the ECP version will be 2010. this may be a problem if you want to manage your environment using the new ECP 2013.
Until you move your Exchange admin mailbox on Exchange 2013, you will not be able to use ECP 2013 EXCEPT if you add the /?ExchClientVer=15 suffix at the end of your OWA 2013 URL (such https://servere15/ecp/?ExchClientVer=15)
In this case ALL of your user (including administrator) will be able to access the ECP 2013.