@ddelaey I ran into a similar problem after 3 different wordpress installations with --wpredis. The solution I found was adding the following to the wp-config.php in each one.
define( 'WP_CACHE_KEY_SALT', 'sitename:' );
replace 'sitename' with the domain if you like. It creates a different key for each wordpress instance.
Another option is to add this two lines, then each site can use a different database.
$redis_server = array( 'host' => '127.0.0.1', 'port' => 6379, );
you can have up to 16 databases.
If you are using HHVM add to your hhvm-redis.conf this line:
fastcgi_param HTTP_ACCEPT_ENCODING "";
Other thing you might want to consider is adding to your redis.conf, if using 64bit os.
see Redis Documentation
hope this help...