Theme not showing

For some reason the theme from my main website isn’t showing up properly on the sub websites, as you can see here:
https://www.pelibar.nl/zalen/ and https://www.pelibar.nl/en/zalen/

On top of that, there seems to be something wrong with the homepage as well: https://www.pelibar.nl/en/

I’ve read trough some older threads, and usually the problem is the htaccess file, but I can’t figure out what’s wrong with it.

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]