Memcached session storage driver error

On the same server? Really? There's three options:

1. It's not on the same server.
2. It's not Memcached, but Memcache (no 'd' at the end; different PHP extension).
3. That server has multiple PHP versions installed, or that other application uses a different PHP SAPI; either way - the one you're using with CI doesn't have Memcached enabled.

Um, actually there's a 4th option - that other app isn't running PHP at all.