Redirect loop in frontend / Subdirectory install

Hi guys,

I have been trying to setup the MU Domain Mapping Plugin (https://www.ads-software.com/plugins/wordpress-mu-domain-mapping/installation/) for my subdirectory WordPress multisite network.

So far, I’ve got the following:
– the multisite runs on maindomain.com
– it has several sites run in a subdirectory, such as maindomain.com/child
– I want to map childomain.com to the site maindomain.com/child
– I installted the domain mapping plugin according to the instructions and mapped childdomain.com to the IP address where the multisite runs

.htaccess of the multisite installation:

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) /wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) /wordpress/$2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ /wordpress/$2 [L]
RewriteRule . index.php [L]

wp-config:

<?php
// credentials and salts...

/* Multisite */
define( 'WP_ALLOW_MULTISITE', true );

define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', 'maindomain.com');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

define( 'SUNRISE', 'on' );

require_once(ABSPATH."wp-settings.php");
?>

The main site works fine now, as well as it’s backend and the network admin. When I go to either childomain.com or maindomain.com/child however, it results in a redirect loop.

I’ve tried various things with the domain itself, including CNAME (maindomain.com), IP address, HTTP redirect…

Does anyone have an idea of how to fix this? I’ve already tried some of the proposed solutions in other threads (altering the .htaccess etc.), but they didn’t work. Any help would be greatly appreciated!