I am bit puzzled and frustrated with this issue I have in my lab. I have exported my test mailboxes from my 2010 environment (other settings are minor and easily reproduced), and uninstalled 2010. Next I went and installed 2013 RTM from TechNet.
The first time I installed 2013 the install was successful, but when the ECP came up it was just a empty browser window - it would never display. So I thought, maybe there was something left over from the previous 2010 server. I blew away the OS (was 2008 R2) and built a clean 2012 server. Installed all the Exchange 2013 prerequisites, extended the Schema and installed 2013 using all the defaults in the wizard.
This time the the ECP login came up, but here is where it gets puzzling and frustrating. I put my Admin credentials in (this account has all proper group membership that I'm aware of) and the screen goes blank for a second and then the OWA login is displayed. The ECP never comes up again, just goes straight to the OWA login. If I enter my credentials in the OWA login, the screen just flashes and the password field clears but doesn't login.
I have tried uninstalling and cleaning up AD and reinstalling, but the same thing keeps happening. I can't help but think it has something to do with permissions, but I'm not sure what that would be.