memcached에 연결이 거부되어서 발생하는 문제입니다. 혹시 사용하시는 서버의 memcached 가 특정한 포트 번호를 요구한다거나 하는 부분이 있는지요?

답변해 주셔서 고맙습니다.
호스팅 업체에서 보안 문제로 기능을 꺼서 생긴 문제였습니다.

디버그 상태에서 로그인 화면에 이렇게 뜹니다.

Memcache::connect() [memcache.connect]: Server localhost (tcp 11211) failed with: Connection refused (111)(8)
File: /chroot/home/html/tc/library/preprocessor.php:122
Memcache::connect() [memcache.connect]: Can't connect to localhost:11211, Connection refused (111)(2)
File: /chroot/home/html/tc/library/preprocessor.php:122
Memcache::connect() [memcache.connect]: Server localhost (tcp 11211) failed with: Connection refused (111)(8)
File: /chroot/home/html/tc/framework/cache/Memcache.php:21
Memcache::connect() [memcache.connect]: Can't connect to localhost:11211, Connection refused (111)(2)
File: /chroot/home/html/tc/framework/cache/Memcache.php:21
Memcache::get() [memcache.get]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/cache/Memcache.php:98
Memcache::set() [memcache.set]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/cache/Memcache.php:101
Memcache::get() [memcache.get]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/cache/Memcache.php:42
Memcache::get() [memcache.get]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/legacy/Textcube.Control.Session.Memcached.php:101
Memcache::get() [memcache.get]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/legacy/Textcube.Control.Session.Memcached.php:63
Memcache::get() [memcache.get]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/legacy/Textcube.Control.Session.Memcached.php:63
Memcache::set() [memcache.set]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/legacy/Textcube.Control.Session.Memcached.php:73
Memcache::get() [memcache.get]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/legacy/Textcube.Control.Session.Memcached.php:63
Memcache::set() [memcache.set]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/legacy/Textcube.Control.Session.Memcached.php:73
Memcache::get() [memcache.get]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/legacy/Textcube.Control.Session.Memcached.php:63
Memcache::set() [memcache.set]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/legacy/Textcube.Control.Session.Memcached.php:73
Memcache::get() [memcache.get]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/legacy/Textcube.Control.Session.Memcached.php:42

...

Current Database Management System :

MySQL
Cache system :

    * Page cache Enabled
    * Skin cache Enabled
    * Memcached module Enabled

Memcache::set() [memcache.set]: No servers added to memcache connection(2)
File: /chroot/home/html/tc/framework/legacy/Textcube.Control.Session.Memcached.php:48