Memcache (and memcached too) phpBB cache drivers rely on exploding (by slash) the PHPBB_ACM_MEMCACHE constant which consists of PHPBB_ACM_MEMCACHE_HOST and  PHPBB_ACM_MEMCACHE_PORT concatenated via slash (see https://github.com/phpbb/phpbb/blob/master/phpBB/phpbb/cache/driver/memcache.php#L55 and https://github.com/phpbb/phpbb/blob/master/phpBB/phpbb/cache/driver/memcached.php#L70).

In the meanwhile, PHP allows using of Unix socket as a host (like unix:///path/to/memcached.sock) with the port of 0, which exploding by the slash gives wrong parameters to pass to the memcache(d) addServer() method making it unworkable.