Ok so first question is are you going to a cas array URL or directly to the cas itself?
I.e are you going to https://mail.yourCompany.com that redirects to one of several CAS servers or are you going to https://exchCasServer/?

Best practice would be to first start off with a specific server and get that to work and thengo trough the rest otherwise if you go to an array or a load balancer it makes things more difficult, especially if trying to get logging and what not

So first off use the URL directly to a CAS server, ensure it doesn't work still, check the Application Logs in the Event Viewer to ensure that no obvious errors are being thrown, past that you should enable logging in the web.config for the mailbox you are testing against and see if it gives you any more information

Also one thing to be aware of is if you are attempting to use the evadmin to open up other peoples mailboxes and shortcuts through OWA, this no longer works and have seen many techs pull their hair out because what used to be a valid OWA testing method in 2007 no longer applies

Oh and also look at the EV servers event logs for anything from Web Application to ensure its not an authentication issue with the EVAnon account, and it may also be worth looking at the IIS logs on the EV server to see if there are any requests coming from the exchange servers ip address and seeing if It is returning an error to them (I.e. 403 etc)