Description of problem:
Trying to view all available authentication plugin, I get the error message:
Authentication plugin cas not found.

Version-Release number of selected component (if applicable):
Name        : moodle
Arch        : noarch
Version     : 2.6.10
Release     : 1.el6

How reproducible:
try to view all authentication's plugin

Steps to Reproduce:
Go to "Site administration" / Plugins / Authentication / Manage authentication

Actual results:
A page with an error message: "Authentication plugin cas not found."

Expected results:
to view all authentication plugin

Additional info:
There is a workaround to view all authentication plugin:
- the authentication mods are in /var/www/moodle/web/auth/
- remove the directory "cas" in /var/www/moodle/web/auth/
- restart apache
- now all authentication methods are available