I have a new Exchange 2013 SP1 server on a 2012 R2 OS where I've installed both Mailbox and Client Access roles in an existing Exchange 2010 environment. Initially ran into some DNS issues that caused errors installing the Client Access server, but I fixed those, removed Exchange 2013 and reinstalled with no errors. In every scenario so far, trying to access the ECP redirects you to the 2010 OWA. I have been scouring threads for hours and tried everything I've seen, but to no avail. I have tried:
1) Using the https://<Exchange_2013_CAS_FQDN>/ecp?ExchClientVer=15 URL, which still redirects to the 2010 OWA
2) Creating a new user account with all of the same rights as the domain admin but has no mailbox, which redirects to the 2010 OWA and throws an error that there is no mailbox.
3) Setting the domain in IIS authentication settings for the ECP on both the 2013 and 2010 server.
Does anyone have any ideas here on what to try next?