I just hate this new web interface of Exchange 2013, its like Microsoft does everything to make your life miserable as much as they possibly can!
So I installed my first Exchange 2013 server (CAS + MBX) which is co-existing with my Exchange 2010 servers. Today I was unable to open up my EAC using my account and it kept throwing up an error in internet explorer saying
"500 Internal server error, There is a problem with the resource you are looking for, and it cannot be displayed"
It turned out that since my user account mailbox is still on Exchange 2010 whenever I went to the EAC url (https://<server name>/ecp) in Exchange 2013 the browser defaulted to the Exchange 2010 ECP and showed me an error.
So in order to work around this you have to go to "https://<server name>/ecp?ExchClientVer=15" and it FINALLY opened up!
Also if you now want to go to your Exchange 2010 ECP you need to go to "https://<server name>/ecp?ExchClientVer=14"