503 Error after configuring memcache

I want to activate memory caching in my config file.
But when I add 'memcache.local' => '\OC\Memcache\APCu', I get a 503 Error on my Nextcloud website.
Without that line Nextcloud works.

The log says > Memcache \OC\Memcache\APCu not available for local cache

I’ve installed the newest version of php7.0-apcu
Nextcloud 12.0
Ubuntu 14.04
PHP 7.0.19