Microsoft Exchange 2013 iApp - Can't login to OWA or ECP if more than one server is active in pool

OK, somehow notifications for these were going to spam folder and just saw people have been asking what happened. Our main issue was co-existence with 2013 and 2010 at the same time. The CAS role in 2013 was not properly proxying requests back to 2010 until the URLS were changed. If you want to test using a HOSTs file before the cutover, the external URLs on 2013 must be your external name: owa.company.com and the internal URL must be the internal server host name: server1.company.com Don't change anything on 2010 yet. When you go to make 2013 your front end for all CAS requests in the environment, you must change the internal URL for the 2013 servers to be owa.company.com, and set the internal URL on the 2010 servers to be 2010servername.company.com. This always the proxy to 2010 to work. In our case, the F5 was hitting the 2013 servers, and a loop was getting created. My main problem with F5 at the time was the engineer was slow to respond to the case initially until a manager got involved. Unless you were already on the iAPP, it's a complicated mess of changing around URLS when trying got use it, and also be within co-existence with a previous version of Exchange. If you are purely a 2013 environment, it's straight forward.