"Error establishing DB connection" when entering random path

When accessing a non-existing path like example.com/random, I get the “Error establishing a database connection” error instead of being forwarded to the themes’ 404 page. How can I fix that?

My client’s running WP 3.9.1 Multisite (subdirectory) with two sites and a “blank” main-site, installed under /wordpress.

“Blank”, redirects to /de:
example.com/wordpress

German site:
example.com/wordpress/de

English site:
example.com/wordpress/en

I’m redirecting all traffic from root to the main site, which redirects to the german site in .htaccess: (of course example.com is replaced by the real domain)

RewriteEngine On
RewriteBase /wordpress/
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]

RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www.)?example.com$
RewriteRule ^(/)?$ wordpress [L]

My index.php says:

<?php
/**
 * Front to the WordPress application. This file doesn't do anything, but loads
 * wp-blog-header.php which does and tells WordPress to load the theme.
 *
 * @package WordPress
 */

/**
 * Tells WordPress to load the WordPress theme and output it.
 *
 * @var bool
 */
define('WP_USE_THEMES', true);

/** Loads the WordPress Environment and Template */
require( dirname( __FILE__ ) . '/wordpress/wp-blog-header.php' );

Any suggestions are highly appreciated, thanks!