500 Internal Server Error for Multisite Subdomain


I'm trying to set up a Wordpress Multisite for a client on http://www.studiopessemier.com. For some reason, the main domain is working, but any subdomains (store.studiopessemier, http://www.store.studiopessemier, etc.) I try to set up get an internal server error.

Some googling shows this may be a problem with the .htaccess file, but I honestly don't understand how, as it's just exactly what Wordpress said it should be. And none of the online resources I found have enlightened me on how to solve the problem.

Anyway, I would really appreciate it if someone could take a look and tell me what might be wrong:

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

# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule . index.php [L]
# END WordPress