Unless you set $CFG->altcacheconfigpath in your config.php, Moodle is trying to create/open a file at muc/config.php inside your 'moodledata' directory. It's not there or it can't open it. It's most likely a permissions issue. If you can't spot anything, switch on Debugging, reproduce the error and then scan your web server's error log carefully. I think it's unlikely that this will be the only error/notice caused by whatever the problem is.