I got it to work... I changed the .htaccess to

RewriteEngine On
RewriteCond %{HTTP_HOST} ^forum.example.com
RewriteRule ^(.*)$ http://localhost:61111/$1 [NC,P,L]

and the config.json to

{
    "url": "http://forum.example.com",
    "port": "61111",
    "secret": "###",
    "database": "redis",
    "redis": {
        "host": "/home/###/.redis/sock",
        "password": "",
        "database": "0"
    }
}