WordPress Multisite issues

I have WordPress multisite installed (and correctly configured according to the WordPress ‘Create A Network’ tutorial) in the root of an add-on domain with two sites (subdomains) created using the subdirectory method of network install.

The end of my wp-config.php file is this:

define( ‘MULTISITE’, true );
define( ‘SUBDOMAIN_INSTALL’, true );
$base = ‘/’;
define( ‘DOMAIN_CURRENT_SITE’, ‘example.com’ );
define( ‘PATH_CURRENT_SITE’, ‘/’ );
define( ‘SITE_ID_CURRENT_SITE’, 1 );
define( ‘BLOG_ID_CURRENT_SITE’, 1 );
/* That’s all, stop editing! Happy blogging. */
/** Absolute path to the WordPress directory. */
if ( !defined(‘ABSPATH’) )
define(‘ABSPATH’, dirname(__FILE__) . ‘/’);
/** Sets up WordPress vars and included files. */
define(‘WP_ALLOW_MULTISITE’, true);

My htaccess file is this:
# BEGIN WordPress
<IfModule mod_rewrite.c>
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]
</IfModule>

# END WordPress

My site structure is as follows:
ADDON DOMAIN: http://example.com/ (wordpress install location)
SITE 2: http://example.com/accountants/
SITE 3: http://example.com/test/
I have been getting random 500 Internal Server Errors every time I try to access the dashboards of either of these sites.
I have a suspicion that the cause of this could be something to do with either the .htaccess file (in the root directory of the add-on domain), or a redirect issue to do with the wp-config.php file. All generated mod_rewrite rules were added to my wp-config file above /* That’s all, stop editing! Happy blogging. */.
Any help would be much appreciated!

[No bumping. If it’s that urgent, consider hiring someone.]