Memcached session storage driver error

Well taking a guess I'll bet that you need to re-install CodeIgniter deleting the ./system folder.

Dude, stop making guesses. If you don't know how to help, you don't have to reply.
The Memcached class is part of a PHP extension.

Thanks for your reply.

I followed your advice and reinstalled the CodeIgniter 3.1.6 by deleting the ./system folder, but the error remained the same.

PS: I use the Memcached (with the path:localhost:11211) for another project (CMS Bitrix24) on the same server, but there is no such 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.