Exchange 2010 Service Pack 2 Rollup 1 has broken some cas servers. I know it sounds crazy that an update direct from Microsoft would do this.
C:\Program Files\Microsoft\Exchange Server\V14\ClientAccess\Owa\14.2.283.3
C:\Program Files\Microsoft\Exchange Server\V14\ClientAccess\Owa\184.108.40.206
Once they are deleted then copy and paste themes folders from a healthy cas server back where they should go on the problematic server.
If the above steps dont work. Go to a server that is working ok and export the following regkeys and then add them to the server that had the problem and re-apply the rollup and problem will be solved.