I think it was not that he can't login into the system. The problem was that it was giving the error "internal script error" when a wrong or empty username is provided when a user try logging in. It should show an error like "Invalid login, please try again" and should redirect the user to the same page.
it was solved when CAS server (SSO) disabled. The only authentication method enabled are email based self-registration and manual authentication.
You can find these option here:Site administration -> Plugins -> Authentication -> Manage authentication