You will find the master list of files that should be under the auth\cas file
Here
https://github.com/moodle/moodle/tree/MOODLE_28_STABLE/auth/cas
Can you check that you have all the files listed there in your install
You will find the master list of files that should be under the auth\cas file
Here
https://github.com/moodle/moodle/tree/MOODLE_28_STABLE/auth/cas
Can you check that you have all the files listed there in your install