Subdomain's slugs showing 404 error

I'm developing a multi-site-multi-domain-multi-server WordPress site :grinning:

It's all working on a dev environment. But on 1 server, I cloned everything on an acceptance environment now.

So the site is: 1 proxyserver sending traffic straight to the WP-server, which has a MYSQL-server behind it (different machines).

The acc is: 1 proxy-loadbalance-server, sending traffic straight to 1 of both (identical) WP-servers, who have one single MYSQL-server behind them

The issue:

The domain acc.domain.com works fine, but any other requests such as http://acc.domain.com/page-one returns a 404 error. But not a WP error, but an apache error.

These sites are not accessible from outside of the corporate network.

The content of .htaccess file is:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [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).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
</IfModule>

# END WordPress
php_value upload_max_filesize 64M
php_value post_max_size 64M

We also tried the .htaccess and it didn't worked:

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

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]

Permalinks are reseted, the DNS are set up and wildcard pointing to the production system .

Please advise,